Two things to look at; nitrate to phosphate ratio and red lighting. This is just my experience and something to look at; while algae scrubbers benefit with red lighting, macro algae reactors many times fall victim to cyano. Most test kits with the exception of the Hanna ULR will show phosphates at or near zero while actually be somewhat higher. If your nitrates are very low, your phosphates could still be too high and create an imbalance.