Im seeing a bit of a bald spot on mine lately and I believe its due to slightly higher than normal nitrate rates. I have hard GSP is especially sensitive when it comes to nitrates.
Alk or ph swings can do that to them. Are they growing and spreading like crazy? If not somethings wrong. Mine never do that and I have 3 different colonies in 3 different parts of my tank. Check param when they are closed and check your test kits exp date. Hope this helps
after or before water changes?
green star polyps.
Kalk in your top off? ATO? Any link to when you hit the bottom of your top off container?