Haha... I think I know what this is.
There are two ways to confirm; post a higher resolution photo, or... Do me a favor, poke it with a stick. Lol.
I suspect these are little beneficial bugs. I often see them in the same places as chitons; hidden under frag plugs and such. They just have a cool camouflage pattern. If I’m right, they will walk/move if they are disturbed. And I’ve never seen them cause a problem.
Otherwise, I think the above are good guesses too. And we could maybe look with a closer photo!