Collonista snail or some sort of polyp

[Dude64]

Just noticed a mystery guest nestled atop my reef. He seemed to "expand and contract" ... and the sort of dissappeared. Only had a brief moment to attempt a pic, so it is what is.

 

[KrisReef]

Harry Houdini, based upon your description. A snail doesn't expand and contract like you describe, so I'm calling it a mystery.

Did you party/rav Friday night?

Somene may have slipped you something that is messing with your eyesight?