Gah, I hate that stuff! It shows up in my shrimp tanks and is a total PITA to pull out. I can't think of anything that'll eat it offhand, either... at least not anything small enough to fit in your tank.
However, I'd definitely cut back on the phosphate dosing, it should lower the rate of algae proliferation. Spontaneous algae blooms are often linked to excess phosphates...
It definitely could be the P... I just started dosing P because the GDA I was getting before is caused by a P/N imbalance (too much N and not enough P)... I'm going to cut back and just dose the traces for a little while. It's tough to find the balance but I will get it somehow...
I haven't tried spot dosing - I'm going to try sucking it out during a water change and trim the grass. The cottony stuff is down at the substrate level so it might be hard to get out during a simple trim, but a trim and a light vacuum during a water change will hopefully suck it out.
IME all vacuuming will do with that type of algae is smush it down closer to the substrate, or clog your siphon. You'd have better luck going in with a stick and spinning it like cotton candy into a ball you can yank out.
I think I got most of it out with my normal siphon - I rubber band a fish net over the tube so that it doesn't suck up the Aquasoil. Luckily it wasn't too entrenched in the hairgrass. I'll post pics in a bit.
I was running 2x T5HO for 8 hours, I think it's too much light and the combination of light + ferts caused the bloom. I disconnected one of the bulbs and I'm going to try 1x T5HO for 10 hours instead... The bulbs are about 16" from the substrate, and I believe each bulb is producing about 38 PAR, although it's hard to tell without my own PAR meter - I'm basing this from Hoppy's data which he collected from someone with the same fixture as me.
I've heard I should be shooting for between 30 and 60 PAR at the substrate. 2 Bulbs would have put me around 76 PAR which is a little high. 38 PAR might be a little low, but it might also be nice not to have to trim every week... lol