I've found similar results after adding 50 ppm Borates. My pool is very high in phosphates (3000 ppb) and probably nitrates as well and is very reactive so if the chlorine gets too low it doesn't take long for algae to grow. However, since the Borates are in the pool it seems far less reactive. It doesn't completely stop such algae growth, but seems to slow it down significantly.

Though not cheap upon first addition, it's better than something like copper ions that can stain. Thank you Evan for getting us onto the Borates bandwagon. It's not necessary since one can prevent algae using chlorine alone, but is very nice insurance to have.

Richard