Does it move? Looks like a pest anemone of some sort, but its foot is on the glass, so the viewing angle is odd. I'm guessing Boloceroides mcmurrichi, but I'm not sure.
Yep, pulses around the mug opening and closing its shell.
hahaha, will try to get this right for you, sorry. That pulsing while opening and closing its shell is a scallop method of locomotion. My guess, and I've been batting a 1000 so far for you, but it's a scallop of some type. I tried to look it up online and in my invertebrate book, but I couldn't see it.
Possibly a hungry flame scallop? Ive read they go white in color when they arent doing so well?