Go to your LFS and get Kalk/lime powder OR find some Mrs. Wages Pickling Lime at Wal-Mart (they may not have it seeing that it is more of a seasonal product). Add a enough ro/di water to it untill it becomes just thin enough for a syringe to suck it up. Now, turn off your pumps and returns and squeeze a bit of paste on the mushroom's mouth so they ingest it. The should die off. Keep an eye on your skimmer and do a water change after they die off.