Algae problem on pool filter sand
I have a 55 gallon tank that I have had set up for around 8-9 months, it has about 2" of pool filter sand in it, 3 pieces of mopani drift wood and some feller stone. I do a regular 20-30 percent water change a week, and have notice in the last few months a algae bloom on the sand, it is dark...I guess maybe darkish greenish blue in color and when I do suck it out of the tank it seems to be almost crusty on top. I have noticed that the sand is getting this color under the sand also, this is not all over the tank only in spots. All the water chemistry is good, I had this same problem while living in Pa. and even there was unable to completely get rid of it, however the bristlenose pleco's helped a bit. My question for you all is could the mopani driftwood be causing this?, also I do not keep the light on very often only a few hours per day, and the water temperature is staying around 78 to 80 degrees. I'm not sure if I should syphon out the whole sand and start over or just replace it with gravel? I really like the sand idea and would hate the idea of transferring media, I would appreciate any advise to help get rid of the algae bloom, Thanks.
1--29 gallon tank: 1 small angelfish, pygmy cory, and 4 baby albino bristlenose.
2--55 gallon tank: 1 adult angelfish, 7 black skirt tetras, 5 yo yo loaches, 1 synodontis catfish and 1 spotted raphael.
3--3 gallon tank with female betta.