Well, in order for coralline algae to grow, you need to have proper alkalinity, calcium, and magnesium levels. I would not add any Purple Up unless you own a calcium test kit. This product will raise your calcium level.
If your rock right now is a slimy green, there may be a problem in your tank. What are your nitrates? Do you use RO water or tap water? If you are using tap water, there is probably a lot of phosphates and silicates in your tank.