First off, there is no such thing as bad algae. All algae help your tank. If you find it unsightly, then find ways to take care of it. I would assume that the purple up may have something to do with this algae appearing. After all, coralline is an algae also and use some of the same nutrients.. Personally, I find the use of any additives unnecessary. How long has the tank been set up? How many gallons? What's the stock like? How long are the lights on for a day? How much water do you change and how often? What's your filtration like? How old are your bulbs? What are your parameters like?