Sounds like it's their guts and that they might have recently gotten injured. That's what happened to one of my mushrooms when someone at a LFS gave me a shroom that was taken from the glass with a razor blade and he did it carelessly, and also when a mushroom came attached to two separate rocks and when I placed it in the tank (trying to stress the shroom the least in the process, of course) the shroom excreted this spaghetti-type stuff like it was being torn in two, but in the end recovered just fine.
So, in my humble opinion, if it's not excessive then it should recover.