Rubble hitchhiker

[TeresaRN]

Hello,
purchased rubble (small live rocks)to break up for my pistol shrimp since he likes to build and close some of his entrances in the rock. I found this button sized soft thing with a white spot in the middle of it. Any ideas?

 

[Oscaror]

Hello,
purchased rubble (small live rocks)to break up for my pistol shrimp since he likes to build and close some of his entrances in the rock. I found this button sized soft thing with a white spot in the middle of it. Any ideas?

Pic is blurry but looks like some sort of discosoma mushroom

 

[Labridaedicted]

Looks like it's growing off some old coal skeleton so it could be whatever died back and left tissue there. It could also be a mushroom as mentioned above.