GSP is tough to get rid of. I use a butterknife and just scrape em off. They will nuke a coral quickly however Xenia is harmless. The only thing Xenia does it overgrow corals and is a pain to control. I have xenia on my glass that constantly sways in a 8 inch diameter green w/ purple rim cap. If I was you, I'd grow xenia by itself of on the glass. That stuff just does not die.