vermetid snails. They are fine. The thread they put out is a feeder, and can link up with threads from other snails to build a 'net.'
They help filter the water, and the used threads get dumped into the food chain.
So, they are fine, unless you really don't like the look of them. You can kill them by putting a bit of frag glue over their hole, or break them out.
One other thing, they are SHARP. careful, as they can cut your hand if you're working in the tank