take baking soda, put a half a cup in a steel pot and cook on stove top until the baking soda boils and moves like water. Now you have soda ash, which has a pH of 10 when disolved in solution, vs pH of 8 if you use baking soda. Dissolve some soda ash into water, measure the pH and once it's 10, fill a syringe with 5 mL of this and inject a little into each mushroom. The mushroom will shrink and die off, but its possible little pieces of it will grow back, so once you inject it, wait a few minutes and you can pull the whole mushroom off the rock with tweezers. It will come off easily.
OH, very important, turn off the pumps when you do this otherwise the high pH water will spray around the tank. Not that it will kill anything, since it's neutralized instantly by the seawater, but it will blow the water off the target. I also kill palyzoas and trim back encrusting SPS by this method.