Kind of white curly stuff?
Hard to tell for certain as the picture doesn't show enough detail.
Sometimes when mushrooms are irritated they expel stuff - I'm not positive what it is, but I've seen it plenty of times with mushrooms newly shipped, fragged or otherwise ticked off.
Usually no ill effects.
If yours (nice Rhodactis, BTW...) has been in the tank a while and just did that spontaneously, I'd wonder what caused them to get mad.
Keep an eye but shouldn't be too much cause for concern. If there's a neighbouring coral or something that might have irritated it, I'd move it or the other coral, but otherwise I'd leave it be so as not to stress it out further.
Jenn
