I've had my run in with this algae recently as well. It doesn't seem to vacuum easily and attaches to my plants and substrates pretty well. I may need a stronger vacuum/suction I guess.
I think high light is definitely a problem. I have a similar setup but I use pressurized CO2 with a drop checker that goes from dark green to yellow/green by the end of the light period, which is about 8 hrs. Riccia and all plants pearl like crazy. I dose modified EI (modify dosing depending on need).
I will try the suggestions made here by Tom. The reason why I got the high light to begin with was to ensure that my HC grows low to the substrate. Would it still grow low with only 3WPG?
I guess time will tell.