This is almost always a phosphate issue but other things come into play. Ultimately you have a lot of bio load for a small tank and things like this can be difficult to solve. Up the water changes to 2g a week and see what happens.
If you want to use something like Rowaphos you really need a fluidized reactor, you don't get near the effect out of the media in a bag. Is the sponge you are using an HBH cut to fit? If so this is an aluminum based product and not a great idea for tanks with soft corals. The easiest way to deal with phosphate is using a chemical that binds and precipitates it like Blue Life Phosphate Control or Carib Phosbuster.
Making this tricky is the fact that low price phosphate kits are all sucky. You need to use something like a Merc kit to get a good idea what is really going on there as organic phosphate is harder to detect. At minimum make sure that the reagents in the kit you are using are as fresh as possible. I use a cheap one myself but I you need to keep in mind you may not have a good picture from it. Also if the source water the company uses has phosphate in it RO won't remove it. You need a DI resin to get it so don't assume it's good just because it is RO.
Alkalinity also plays a part in this. You said your pH is 7.8 which seems low for a tank with a skimmer. Do you test for this? Try and keep it at the high end of the safe range (200ppm / 7dkh / 4meg/l) hair algae hates that.
Another thing that would help are getting a bit more flow in there. Swap the power sweep for a Korallia 1 or Sieo 620.
I also noticed from your FTS that your open brain isn't showing much polyp extension. If it's usually like this try putting it on the sand, the rock rubbing on the polyp may be bothering it.
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