What on my mushrooms mouth?

[Cornflake]

My mushroom has been happy the past few days and I found this on it tonight. Any ideas?

 

[Tahoe61]

Mesenterial Filaments, an internal structure. Corals may display filaments for a number of reasons including stress. I have seen corallimorphs do this a number of times and I never noticed a decline in the coral. Sometimes it's a feeding response or a defensive mechanism.

 

[danieyella]

My acanthophyllia does this whenever a snail falls onto it and walks around near its mouth, luckily, it has not suffered longterm from it.