Stillness is not being especially still. Not trying to fight it, most physical sensations are worse if you try to fight them, but that means he's not tensed up and he's not hiding how terrifying this is either.
They say the worst part is feeling your tail wither up but Stillness thinks it's the legs forming, how new and part-of-him and not-part-of-him they feel, and the sun on him without water in between and the drying-up feeling that he can't get out of and can't flinch back into the water to get away from
He is acutely aware that he's being watched but it's not like it's strange or remarkable to find your second transition terrifying, it would be strange not to, he can't tense up he can't tense up that will make everything so much worse.