Black algae info please.

A friend of mine said tonight over the phone that in his 75gl reef the brown algae is turning black after he started dosing kalkwasser. I haven't seen it yet and have never seen brown turn black. I guess the brown was diatome. He has had a problem with algae since starting the tank and I have tried to help, but for some reason either he doesn't hear what I'm saying or there is something going on that we can't find.
He is using RO, Coralife's largest superskimmer. 4x4'VHO's, 18w turbotwist UV, Emperor 400, Magnum 350 and 4 Maxijet 1200's. He says ammonia is 0, nitrate 0 and phosphate 0. The tank is about 3 months old, maybe 4.
A few corals, about 75-100#LR and 3" sand bed.
Any ideas? Is the kalk killing the algae and turning it black? He says most of it is on the sand.

 

If its black slime then its cyanobacteria. I had the black cyanobacteria before and it not easy to get rid of, a big pain in the a**.