How long has your sand bed been going? Did you go with a deep sand bed? The reason I ask is I have read and heard from other reefers that over time our sand beds can store nutrients in them (phosphates, nitrates, organic). Then after a while, they release them back into the aquarium. Maybe the algae is growing on the sand cause what it needs isn't in the water but coming from the sand. The nutrients leaking might just be low enough that you cant test for it, or it is getting used up as fast as it is leaked out. This is why I have had a Bare Bottom for years and this is my first tank that I have had an algae problem. Only to find out that it wasn't my husbandry but but an algae that I broght in on another rock. Bryosis.