I have had the same stuff growing on the underside of my oldest LR(about 2 yrs old).
Its not a big deal to me. The top sides of that same rock are completely covered with coraline algae. If it is a sign of bad circulation I just licked that problem. Bought a Dolphin 2000 for a 55 gal and 30 fuge. Almost overflowed my friggin tank. LOL
I also just checked my PHOS and it is 0.0. Although my nitrates are around 2.
Also the only time my protein skimmer really pumps out alot is when I put my hands in the tank. Is that a problem? or do I just have enough LR/LS/Macro to keep the Dissolved Solids at bay?
I havent noticed the stuff growing anywhere else in the tank and knock on wood it doesnt become a nuisance. Good luck with the stuff and if you find out anymore please post.
Themadd1