Coral sponge or nem? ID

[at11ok]

I found this behind one of my rocks today and was wondering if anyone knows what it is?

 

[Wen]

Mushroom

 

[MaxTremors]

Discosoma sp. mushroom, soft coral (technically not a coral, it’s a coralimorph, but colloquially it’s a soft coral). I tend to think of mushrooms as kind of in between anemones and corals.

 

[at11ok]

Discosoma sp. mushroom, soft coral (technically not a coral, it’s a coralimorph, but colloquially it’s a soft coral). I tend to think of mushrooms as kind of in between anemones and corals.

Is it common to find them when I didn’t put it in there?