help, I don't know what this is

[ErikHrysyk]

I saw this weird-looking thing earlier today, it's clinging to my wall and isn't moving,(it looks like a yellow sea cucumber. does anyone know what it could be? is it a Aiptasia?

 

[Rtaylor]

Pic isn’t very clear. A sponge maybe?

 

[ErikHrysyk]

Pic isn’t very clear. A sponge maybe?

we didn't put any sponges in the tank though, only fish, 2 starfish, 2 small cleaner shrimp, and a sea urchin

 

[ErikHrysyk]

Pic isn’t very clear. A sponge maybe?

this is the clearest picture I
could get

 

[WheatToast]

we didn't put any sponges in the tank though, only fish, 2 starfish, 2 small cleaner shrimp, and a sea urchin

Perhaps it could have hitchhiked on the rock (assuming it was not dry). It does look somewhat like a sponge, though I am not sure.

 

[ErikHrysyk]

we had someone come over to test the water and when I showed this to him he said it may just be a bristle worm.

 

[ying yang]

This image and your image dont look very similar to my eyes.
Not even the slightest lol

I would go with some kind of sponge also with your picture.
Is it in a low light area of your tank by any chance ?
And you say in first post it's clinging to your tank wall and not moving and looks like a yellow cucumber so guy who test your water say it looks like a dead or alive bristleworm just for curiousitys sake ^_^