There are sitting to high on there mounts or somthing is annoying them. ie crab, shrimp, fish they will do that to protect there foot from harm
Example: pluck one off and place itr on the tip of your finger keeping in mind it has no bones and supporting structures. It would do the same thing it's doing now. sooner or later they will fill will water and flatten out or even fold the other direction into a cup shape. they look heralth let them be for a while and keep us updated see what they do.
Can you get a better shot with brighter lights??
maybe of the foot to ?
How long have you had them ??
Have they been moved from a diffrent place in the tank ??