I've got great news guys!! The problem is going away!! Today there is very little left of the white stuff. *happydance*
I did a sort of "experiment" to see what was responsible for the growth. I pulled the plants that were most covered in it and put it in a fishless/flowless tank. Overnight the white stuff on them disappeared. Because of this, I'm thinking the white stuff is fed by either ammonia, nitrate or nitrite, for in the absence of the big three the stuff disappears.
(I'm thinking the nitrate might be the main culprit, as when I returned from vacation the nitrate level was higher than usual (~25 ppm), but ammonia and nitrite were still zero. And after all the water changes brought the nitrate level down to near zero, the white stuff is disappearing.)
I definitely want to get a bigger tank for these girls. IMO the tank seems over stocked, especially since the girls aren't full grown. With the bigger tank the girls will have more swimming room and the levels will (hopefully) be more stable.