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?