You need a crank bolt. Any bike shop will be able to sort you out, even Halfords do them for a few quid a set. Though make sure your crank arm isn't buggered. While you're there, buy yourself some chain lube nd a new chain.
i thought the cranks looked a bit twitchy too! on second looks though it could be grease or thread lock. i suggest cleaning the surfaces the meet i.e. the splines and inner crank arm (if it has grease on it, i may be wrong)
