You don't - any plug thats been in a system any length of time, is going to have growth. In a mature, low nutrient system that may be a few diatoms and coralline algae. In a higher nutrient system, your going to have issues with hair algae, which is what I assume you're referring to. Hermit crabs, snails, and other members of a clean-up crew will help with algae issues. If you're seeing white, clean plugs, that means the frag is new.