The waste is water that didn't make it through the ro membrane but has gone though your particulate and carbon filters. It should be good for most anything.
I had zero algae problems in my pond. I'm pretty sure Jerry(JNG) used to do the same. The only time I would have algae is after the thaw and before the plants grew. It did that regardless if I used RO/DI waste water or not
I had algea problems in my pond even before I started putting the waste in there when i have too much to use around the house. We added a sterilizer to our pump line and cleared up most of the problem.
Waste water is cleaner then tap, at least on my system, my tap registers 40-50 on TDS the waste registers 15, maybe since its the rejected RO membrane water there is other stuff in it I don't know, I only know that it worked fine in my piranha tanks and did not promote algae growth. Do you have a TDS meter ? If so get some readings of what the waste is putting out and what your tap registers at