Even though it's most likely steam, it doesn't necessarily mean it's nothing to worry about. If as Don mentioned it was just condensation, then it will be there sometimes, like mornings and cool days, and not there other times. But another cause of steam can be a restriction in your cooling water flow, allowing raw water to flash to steam in your mixing elbow. Does the exhaust "sploosh" the way it should? If you start seeing it more often, I'd advise keeping an eye on it, and maybe check your impeller and heat exchanger.