Try taking that cap off and tighten the crankbolt that might be hiding inside?
No, it's probably a crankbolt with an integrated dustcap. They're great, if you have a hollow spindle BB it's totaly worth replacing your old 14mm bolts with 8mm allen head bolts with dust cover. No more having to take out teh dust cover, no more having to worry about having a 14mm socket that isn't going to strip the bolt head. It's pretty great.
Here it is installed:
and here's one so you can see what it looks like out:
