Imo its seems to be just nutrient build up. Vac the sand and to a 20% or so WC for the next 6 weeks.
Ugly truth You'll see some improvement in 3 weeks. In 8 iltll be gone. Don't change anything else, but make sure you are feeding adequately.
Some prime weekly Prob wouldn't hurt.
Imo you have a pretty good bacteria going if your not having other crazy algae going on. (maybe too good?)
Sounds like your on the right track in beating this down.
Id grab a lux meter to see How much light you have.(search lux here on the site.) Increades photosynthesis reduces PO to have your corals do the same work your chato is doing by finding out if you can up the intensity.
free
Galactica luxmeter for Iphone
LUX Light Meter by Chrunchy Bytebox for android.
All around best most accurate is still a hand held Lux meter. $15 for cheap $75 for a milwalkee.
if its excess bacteria adding may not be good. so check. Prime and refresh may be similar. Seems like prime on the west coast and DR t on the east.