Fun Fact: due to a selkie's pelt being a significant part of them and Very Much a part of their body, it does have some limited range of motion even when not actively being worn - not really a whole lot, mostly just a "you leave the trunk with a selkie's pelt in it out for a month or so and when you finally check it again you discover that the pelt has slowly begun to crawl its way out the crack between the lid and the body" way, rather than anything actually detectable on a reasonable timescale, but it Does exist!

Mainly, this is displayed when taking a pelt on or off - a selkie's skin needs the contact with an autonomous body to draw the energy to be truly Active, through the same mechanism through which it can merge with someone to allow shifting to selkie form. While taking a pelt on or off, it'll shift a bit to align itself to your body - or to merge itself, in rare cases.

This looks... generally, Very Bad to outside observers. Seeing a pelt suddenly gain a life of its own to wrap around your body and shift it into a certain shape? Bad To Look At! It doesn't help that pelts have a very fluid and "amorphous" way of moving, or that almost all species a selkie could shift to are a lot more soft-bodied than your average bug.

All of this makes it look a bit like the skin "eats" the bug, and then dissolves their chitin to force them into its shape, and the uncanny valley effect is a whole lot worse when it's not a pelt that's Yours - a pelt that isn't your own tends to fit Wrong, and the handful of structural bones on the inside of a selkie pelt will fail to merge entirely, which can cause... very bad effects, from the perspective of the bug looking at it, not to mention the one "borrowing" the pelt.