Tough one

[Ninjafish40]

I have this thing in my tank and unfountly it's crazy fast so I can't get a shot off but at least at the moment it's about half the length if a pencil ereaser half as much big around and milky white colored

It came out of a small cave grabbed a tiny chunk of food and dashed back into its hidey hole

And guesses ?

 

[Regan_tyler]

It's a little shrimp that traveled in on some live rock had a few of them in my old tank.

 

[Ninjafish40]

Huh I never woulda guessed that