aiptasia?

[Gengivo]

Is it possible that aiptasia are sprouting directly from the stalk of zoanthus?

 

[Tahoe61]

Yup.

 

[EnterName]

That very much looks like (Ex)aiptasia sp.

If you can take out the rock for a treatment, I would do so. As there are zoanthids, you want to wear eye protection and preferably gloves too. You can use kalk paste or specialized products like Tropic Marin's Elimi-Aiptas or Arka's Aip-Away to dissolve it. Once it's fully dissolved, rinse the spot well with some RO/DI and return the rock to the tank. Your corals won't mind a few minutes out of water.

If you can't take out the rock, I can recommend putting glue over the whole thing. If that's not an option, use one of the mentioned products according to its instructions. Usually the flow has to be fully turned off during and after treatment for about 20 minutes, so the anemone can be fully dissolved.

Don't use boiling water in a syringe, don't use citric acid and don't try to scrape it off. I know some people report great success, but these things can re-grow from tiny fragments of pedal tissue that survive. Some strains appear to spread far more aggressively than others, so just because it worked for some people doesn't mean it will work for you as well. Even the mentioned products can't guarantee 100% success.