Thanks for the PM. As noted above, I'd remove the mushroom - it should do fine - and anything else you want off of the rock.
Take the rock, and put it in freshwater. Outside is fine. Add some bleach to the water, and let it sit for a week. Feel free to scrub the spot on the rock. I prefer bleach, because it dissipates in the air. Then run lots of water thru it, changing it a couple of times a day. Depending upon how big the container is, take a pound or 2 of anhydrous baking soda (bake it at 350 for an hour or so), and sprinkle it LIBERALLY over the rock - especially where the bryopsis was. Leave it for a couple of days or a week. Then do some water changes regularly.
Depending upon how big the rock is, you may not want to add it back to the tank, before recycling it. You can then add seawater from your tank, and a handful of sand, let it cycle for a couple of weeks. Then add it back to the display.
Having ruined a tank that started with a tiny spot of bryopsis, I would nuke the rock RIGHT NOW.
Bill
215g FOWLR... and anemones, GSP, gorgonians... carp, that isn't FO!
"I killed my first SW Fish in 1971..."