Take a picture if you can, or search the boards for threads with pics of aiptasia - you definitely want to find out if these are aiptasia or just hitchhiker feather dusters (feather dusters are good, aiptasia is bad). If this is in fact what you have in your tank you do want to try and get rid of them for 2 reasons:
1. They spread like crazy, and although they do help with water quality to some extent (they're filter feeders and take excess nutrients from the water) when they start taking over you'll wish you hadn't let them get to that point.
2. If you're keeping coral or other anemones, aiptasia will start to sting these other inhabitants, likely killing them in the end. With the low cost to rid them, it would certainly be a waste of money to let them go and kill your coral.
There are a few ways to get rid of them. You could squirt Joe's Juice or boiling water at them, or if they're small enough you could try a natural method such as getting some peppermint shrimp. There are other fish/inverts that eat them, but peppermint shrimp are cheap and easy to keep.
Hope that helps.