From what I can tell, I believe its poop. Nothing more, nothing less. Mine do it on a relatively routine basis, seem unnaffected. They're about 1 year in tank now.
Most mushrooms can expell their mesinterial filiments (I spelled that wrong, but it just means their guts) in response to being bumped or moved or just anything in general they don't like. These filaments like like thin white lace.
They will pull them back inside in a little while.
Thanks to all who responded, my mushrooms seems to be doing okay so I guess..... no big deal huh? Well another bit of knowlege under my belt. It sure is nice to have found this group, and the info. that is shared.