He feeds the baby! The greens take so many pictures.

Mermaids are presumably also taking many pictures, but since their cameras are built into their suits this is less visible. 

Fed babies curl up and go to sleep on their attached adults. The mermaid in the water conveys through translation by their land-suited compatriots that when the grey wants to go back on land the baby can in fact be detached safely. 

The grey eventually does detach the baby and pat it and send it off to its parent.

The parent carefully attaches the baby to their tail and waves at the grey as they leave before swimming back to camp. 

Eventually, between the two groups, they're sure to get enough vocabulary accumulated for machine translation to start working reasonably well sooner than later. 

Machine translation is much better at writing than at sound, especially sound with phonemes it wasn't trained on; what have they got in writing?

Their suit projectors can totally do writing! They have a lot of random books in their suit computers! 

That's great! They can feed those into their own computers and get written machine translation working on it, with some bugs since they don't have anybody legitimately fluent in both to push patches.

"We are delighted to meet you! We don't think our name for our species can be rendered in your language," writes their computer.

"We are also delighted to meet you! We have devised a method for rendering our phonemes into your writing; it isn't exact, but by representing our hard phonemes as some of your consonants and our soft phonemes as vowels we think we have devised a way to transfer our speech into yours with better-than-random correlation! We are, then, the Keoatiek."

"We are the Amentans! The green handling the linguistics will say that aloud now."

"Amentans," says the green.

The mermaid computers record this. "We're probably going to call you aliens or land-walkers, at least for a while," the lead mermaid admits. "We can't pronounce that very well." 

"That's okay. Keoatiek isn't very easy to say either. We have stories about imaginary creatures that look sort of like you called 'merpeople', which means people who live in the sea, and we might sometimes call you that."

"That sounds totally reasonable!" 

One of the other mermaids points to an Amentan's hair and says, "The colorful things on your heads, are they hats or part of your body?"

"They're parts of our body! The colors signal which caste we're part of."

The mermaids confer briefly. "Caste? That's not translating very well."

"Amentans have six castes. We inherit them from our parents, and they let us know what sorts of jobs we will be good at. It's more complicated than that, of course. The caste we didn't bring an example of is orange. The ones we have here are blue, grey, green, yellow, and purple."

"Oh! That must complicate adoption a lot, but I think I see. It would make eugenics a lot more precise, too, wouldn't it."

"It does help with eugenics! We don't have a lot of adoption but when we do it's within castes."

"...You don't have a lot of adoption?" 

This seems to concern the mermaids. Like, a lot. 

"Most people prefer to keep their own children and arrange to be able to do that. When there are children whose parents die usually their family members will take them in."

There is more discussion among the mermaids. 

"How large are your clutches?" the leader finally asks. 

"- we usually have one baby at a time, sometimes two."

"Oh. Yes, that would result in much less adoption! We lay clutches of hundreds of eggs at a time, and we usually can't find adoptive parents for all the ones the birth mother doesn't take home."

"Do many of you lay eggs?"

"--We're not biological hermaphrodites, if that's what you mean, and we sterilize for all major crimes, and have a number of compensated voluntary sterilization programs, so less than half of everyone who survives to adulthood has biological children at at some point."

"That would still multiply your population - enormously - every generation," say the Amentans, after some frantic whispered discussion.