So one of my astreas has this growing from its shell. It doesn't really have a visible base, and it doesn't look like a vermetid snail, but I'm extremely inexperienced with this. That was the best picture I could currently take.
could be a spirobid worm or vermitid have the to see the backside base. Vermitids make a spiral, if no spiral then probably something else. I bet spirobid.