Some common denominators I've noticed often with people having this issue is that the tank parameters are typical for reef keeping, the tank inhabitants are doing fine...and the salt used is RC. Since this doesn't happen to the majority of RC users, it *may* just be a bad batch (possibly too high a vitamin concentration, perhaps?).
About seven months ago I had around 80% of the entire sand bed covered daily, with 1-2 NO3 and 'untestable' levels of PO4 (Salifert) using RC/IO blended at 60/40. After a few months, as a test, I used this RC/IO mix with 50% RS Blue Bucket. After a few weeks I definitely noticed less of this stuff on my sand bed and over the last few months it's stayed at a consistent level (maybe 30% less than your photos). My RC/IO mix is just about done, so my next step is to use RS Blue Bucket and IO together (50/50) and see if there are any improvements.
I can't say at this point that RC is the culprit, but the bloom manifested itself just a few weeks after I started using RC, so it is a suspect.