Not very easy to tell without a photo, but I would assume that it is either a hydroid, a majano anemone or an Aiptasia anemone, none of which are things you want in the tank. Remove the actual parts of Halimeda that this invertebrate is attached too and take it out of the system.
Very few animals really eat Halimeda, parrotfish come to mind, but they are not easily kept in home aquaria.
>Sarah