A small bit of rust wouldnt be enough to cause a problem. Ferric oxide is what they use in phosphate removers such as phosban so I would think a little bit of rust would be ok unless it was copper in that case all your inverts would be dead. I wouldnt recommend leaving chunks of metal in your tank but I dont think it will do any harm. Most pumps have screws that are exposed to saltwater and will rust and I havent seen any ill effects as Ive also found a rusty screw in my tank with thriving coral. Now ive seen hermits flip snails and eat them which is why I dont carry too many of them in my tank.