What kind of water flow do you have? I aim my powerheads to just slightly graze the top of my sand just enough to create flow, but not enough to stir the sand up. You can also get some nessarius to keep the sand stirred.
How old is your tank?...i also have a FOWLR,not a single invert ,,,haven't had any kind of CUC in years..may be something you need to wait out....reduce your lighting schedule,,,no corals means you can run your lights only an hour or a 2 a day...
im pretty sure its just a stage of aging but its ugly, but idk how to reduce it, but yea its fairly new, just a snowflake in it right now my angel is in qt