yellow polyps

I had yellow polyps in my old tank, but then eradicated them (i had no space for them and wanted xenia to spread) to leave only a few of them on a frag plug, which i put into my "trash can" tank(i don't have a sump so i use a small tank to drop unneeded stuff into), which had wery little light...

The battle with GHA is turning toward victory. The last coral to be cleaned is a small section of yellow polyps. What’s the best method? Is there a risk of poisoning the tank if I’m too aggressive with it. Sorry if these are rookie questions.