Besides nutrients, pest, bacterial infection, flow and light could be fish or snails. I have a flame hawkfish that tends to rest on them so they close up. Like I said I had a plug covered with 3 different zoas. It opened when I first put it in tank. Then closed up, had dinos, zero nitrate .03 phosphate. Added some brightwell neo nitro to bring nitrate up to 3. Dinos dissappear almost overnight but then nitrates were 40 and phosphates were 3.0. Been doing water changes to bring it down. Now at 20 for nitrates and .2 for phosphates. No algae so not worried about levels and zoas have been doing well, took 3+ months, half covered plug is now twice the size of plug. Working to get nitrates under 10 and phosphates under .1.