Generally considered bad, yes... Killing it is what I'd do, but be careful... If you just pull it out it can regrow, and any little bits that break off and float around can regrow... Do some searching on Google, here and ReefCentral and you'll see 100's of ways of controlling Aiptasia... Personally, I used pure lemon juice in a hypodermic needle and injected them with it, and also got some Peppermint shrimp to eat the smaller ones, but that's only one of many ways of controlling them...
Also, if you've got one, there's a good chance there are others you haven't found yet...
Not trying to freak you out or anything, some people actually don't mind them, but in my experience they tend to spread quite readily, so I'm pretty quick on the trigger to eradicate 'em when I see 'em...