I don't remember ever seeing diatoms in my tank in a layer like that. I have had Cyanobacteria in sheets before. Here is an article about different algaes:
http://www.lenntech.com/eutrophication-water-bodies/algae.htm
It says that cyano can be brown said:
I should have said "a 20% water change after each 2 day treatment", instead of "2 20% water changes after each treatment". And turn off skimmers and phosban reactors. Be ready to skim tons of liquid after you finish treatment. It will overflow your containers. I ran an air pump to a wooden air stone in each tank in my system while treating. This can't be overemphasized, as Chemiclean is an antibiotic that depletes oxygen from the water, drops the pH, and without aeration, loss of livestock could occur. I had to build shields for the tops of the tanks to keep the water from getting on the lights, and making a mess from all the bubbles. But it was worth it to be free of cyano!