maybe a barnacle or small clam. they can move, they just don't very often to conserve energy

Maybe it's a sand crab?!

Okay so my friend went to the beach and is scared about what critter he caught from there. He say's its inside a plastic cup he has over it and that it makes moving around sounds and scratches the cup every now and then. He wants me to guess what it is but I'm lost. He say's it's:

White

Is small

Has a shell with lines on it

(here's the weird part) the 'thing' opens when it's in water and closes when it's out

I'm like so confused HELP!!