Fluorescent green mushroom question

I have a fluorescent green mushroom in my tank that I have had for about a month now. The tank was well established for over a year, all levels are good, great growth from my other coral. My question is I have some white "stuff" coming out from under it. It kinda looks like a very tiny mushroom but Snow White. Any idea what it is? My button polyps have doubled in the past month, from 6 heads to 12 and my green star polyps have also spread quite well. But the mushroom just stays the same. Small in the dark and 4 times that size in the light. How do they grow more mushrooms?