I had this and it took me a while to figure out it was mustard algae. These were my symptoms: it grew in shady areas, on a step, on the walls, on a shelf. It was green/yellow color. It would easily brush off only to return in a few days. It seems very resistent to chlorine, I would shock and it would die but then return. It is slimy to the touch. I dind't have a huge chlorine demand with this and my CC was 0 even with the mustard algae. No bloom, the water was pretty clear. I had this problem for quite some time.

Here is how I got rid of it.

Put in a quart/bottle of polyquat 60. Brought chlorine up to 25 ppm with liquid chlorine 12% and held it there for 2-3 days. Ran the pump 24/7 for these 3 days and brushed once or twice a day in the trouble areas. Keeping the chlorine high I think was the key. I tested 2 or 3 times a day and kept it up with the bleach/chlorine. After 3 days I let the chlorine level drop to normal then after it dropped to normal range put in another quart of poly quyat 60. Cleaned the filter. It's been gone ever since.

good luck