Looks like cyanobacteria (a.k.a. blue green slime "algae"). You need to figure out why it started because eliminating the cause is the best way to get rid of it. You can use erythromycin but they say it can kill your biofilter (although this has never happened to me when I used it).
Remove what you can physically. It comes up in sheets. How much circulation do you have? No plants, right? What is your nitrate measurement...this is a time when you don't want it to be below 10ppm.
The tank has decent flow from canister filter and 2 large air stones, but I have been considering putting a powerhead in there. Will that help? As far as the chemical you recommended, where would I get that if need be?
There are no plants, Nitrate was at 20 before I did my water change. I'm letting the new water run run through and mix a little longer before I retest. I have an extra bristlenose pleco I can stick in there if he'll help?
Erythromycin is Maracyn, available in fish stores. API also sells it as erythromycin. Remember it may harm your biofilter. You can remove it. Just keep trying things until you are sure it is not coming back.
I really thought I was at the end of cycling, as I have been dosing with Ammonia and getting readings of 0 Ammonia, 0 nitrite, and about 20 Nitrate. I was actually going to move my fish over tomorrow. I noticed the growth right before/as I was doing a water change. I was going to dose again tonight, test tomorrow, then move, pending his results again. This will be 3 days with Ammonia being processed effectively.
I'll keep an eye on the algae and if needed, I can run up and buy the erythromycin.
Your GPH is low, only 4.4X when 8X is at the low end of ideal IME.
I would get another canister the same size as the one you have now and then possibly a powerhead as well but that can wait a little.
Remove the algae manually before adding the fish, and continue as often as possible, at least weekly. Remember is it actually bacteria, not algae. Not all types of cyanobacteria are harmless to fish, although I have never personally experienced a fish being harmed.
Ok, I thought that at least 4x was the magic ratio... I'll work on getting an additional canister.
I don't think it was blue green algae though- after doing some looking, I think it was white fungus. It was on my air stones as well. I cleaned the gunk off everything and am keeping an eye on it now. It may have been both, I just don't know...