do you mean the purple coraline algae or other green stuff? really its all personal choice. some people love the purple all over the glass and some dont. I scrape it off, i like clean glass. If you mean hair or other algae a lot of people spend their life trying to get rid of it while others have quite a bit in their tank. I'm ok with a bit, i think its a natural thing to have a bit of algae in your tank but other people feel like their failing their reef if they have some. Idon't like it on my coral or in very visible spots but I have some on the back of my rockwork and a little patch here and there and thats fine with me. if it gets too much i take care of it. The main thing about having some is it means your water either has phosphates or nitrates and you shouldn't have much of either.