I updated your signature, with your new data. Please note the gallons figure: it's based on a 20x40 pool with a 3.5' WATER depth and a 9' deep end (8.5' water), with a break at the midpoint, and a hopper bottom. If the break is closer to the shallow end than that, OR if the deep end is straight sided with right angle transitions from the walls to the floor, then the volume will be higher.

+ If the algae is green, you need higher levels of chlorine. If the algae is BROWN, then it's dead, and just needs to be filtered.
+ Do NOT backwash too much. Backwash when the filter pressure has increased 5 -8 psi over the clean pressure. (You do have a working pressure gauge, right?)
+ You can TEST your filter with a small amount (~1 lb) of DE powder added directly to the skimmer. If it shoots back into the pool after going through the filter . . . your filter needs work, and you may not be able to remove the algae till the filter is repaired.