There's a shelf which once held books, and among the mess on the floor, some of those books can indeed be found. It takes some searching but the relevant manual can be found partially underneath a screen that has fallen off the wall when part of the wall broke.

The manual is made with those plastic pages which are popular for reference material in areas where one can get greasy, so it's survived being underwater quite well. Though the pages stick together some and make a wet sound when pulled apart.

He's just about found the correct area in the index and is half a page through reading when something warbles behind him.

The Tender looks about the space and sit down on the floor. The red of their eyes pierce the darkness, which is everywhere outside the light cone of the torch.

They have learned that a natural topic of conversation is food related to the conversation partners apparent interests.

"There is a species of creatures who share some characteristics with what you'd know as starfish. They eat a kind of kelp shaped somewhat like tomes such as that one."

They briefly point to the manual, because pointing is an important building block of communication.

Yupp, fishlady - fishlad? fishperson? is back.

"Fascinating! I wonder... what do you eat?"

"I no longer require material sustenance. I am sustained by my proximity to Invi'ctmanis." They gesture downwards. "It is delightful."

They step up next to Mark and peek at the manual with lithe steps.

"Tomes are for recording lore, are they not? What secrets are contained within? If it is advisable for you to share, of course."

"Don't you miss eating? I think there's a lot of community that happens around at the mess hall. Actually, are you a social species at all?" If not then they're probably even more alien than he realized so far.

"Oh, this one doesn't contain any secrets, it's just the service manual for the A.S. 412." He points at the reactor. "I'm looking for the disassembly checklist, I fear some water got into it."

"There are no others like me. I tend to the depths, and this is my purpose. I communicate to fulfil the needs of the beings in the depths and to care for the ecosystem." They say it simply, without sadness or other negative affect.

They look at the reactor. "What is an A.S. 412. What is its purpose?"

"Where do you... come from? Like - where you born, created, formed?"

"It's short for Atom Smasher - revision 412. It... smashes atoms? It's a fusion reactor, it uses the same reactions that happen inside stars to create heat which is then converted to the other forms of energy a starship needs."

"I was created, or likely transformed, to what I am. I woke up with great clarity of what I'm here to do, without memories of what was before, several days Gillward from here."

"It is a source of energy! I understand. Would you like to entreat the help of the Membrane for the disassembly?"

So, on the one hand, slimy biological stuff next to the reactor feels wrongbad.

On the other hand he is slimy biological stuff, and he was planning on disassembling it...

"Maybe? I think it could be helpful, but I worry about slime getting inside the reactor? Although to be fair, if water already got into it slime is not going to be much worse..."

"The membrane could dry itself of its water, if instructed to do so! Here, let me show you how to attune to it."

Knock knock!

Come on in, it's open!

Mark accepts the connection.

The Tender walks into Mark's mind, again careful to only look at things indicated by Mark. 

In case "it's open" is literal, the Tender makes a show of carefully shutting the door behind them, with no indication that they're being superfluous.

The Tender walks to a wall of the command center, which obligingly dream-shifts into a space station-style porthole, about a meter across.

They beckon Mark over and then point out the window, because pointing is an important building block of communication. 

Outside, there's a tendril very similar to the one that appeared when Mark was struggling to open the door to the space ship, slowly floating through space against a background of stars..

"You must be careful to only reach to known recipients. If you reach into emptiness, without a recipient, there are a multitude of beings that may reach back."

"The Membrane is safe though, and while it's not very clever, it's eager to help. It will not harm you unless you directly instruct it to do something it does not understand will harm you."

"You can also reach toward me, though it would be a mistake for me to let you in. I will sense you seek contact, and come to investigate. I may be too far away, and if so, you may ask the Membrane to pass me a message as previously mentioned."

"That out there is a symbolic representation of the membrane. Try reaching out to it specifically."

How does Mark conceptualize reaching toward something outside his mind?

He tries to... uh. Imagine a ghost-like hand, that passes through the wall of the spaceship, and then tries to poke the tendril.

Poke.

A ghost hand indeed appears, and when it touches the tendril there is a flash of the presence of something else. It's large, writhing and eager.

The tendril wakes up, flips to turn around, and stretches towards him. Almost insistently, like it wants to say "Come on! Let's play! It'll be fun!"

"That was you knocking, and it accepting. Now you're okay to push into it."

The Tender gestures imploringly.