Looks like Zebra Muscle

[King_Aero]

Take a look at the attached picture. The item in red is what I'm looking to get ID'd. I have no clue what it is or how to begin to search for what it might be. This is the first time its shown up. Hopefully, it's a good critter!

[stevelkaneval]

it looks like some sort of muscle i have a couple in my tank

[ajx22]

Misc. bi-valve of some sort; common as hitch-hikers on live rock.

Most don't survive; but look kinda neat.

[j2quinn]

zebras would only be found in fresh water. but like aj said definitely a bi-valve . It is a filter feeder if you wondering.