Thanks!
Before reading your reply I attempted to remove it by picking up the piece of rock and scraping it off with a pocket knife, in a small Tupperware container of tank water I then poured down the sink. It had retracted completely and I couldn't really tell if I was getting it - I did scrape a bit of material off the rock not far from the mushroom but couldn't tell if that was it or not. Then I saw what looked like it on the nearby live rock. Could be another one, or maybe the same one had let go of the piece of rock the mushroom was on when I picked it up? Anyhow I sucked up the one I saw on the live rock with a turkey baster and shot that down the sink. Both (if they were not in fact the same one that had moved) were only about 3-4 mm in diameter fully opened. I noticed that when the one on the piece of rock the mushroom was on was open that the mushroom would retract away from it.