With corrosion, nothing is simple; as such here are two answers:
Warm/hot water is leaking from the connection and evaporating; it looks like there's some copper by products so something is corroding; is it raw water after the heat exchanger on its way to the exhaust elbow? The water flow may be restricted, so temperature and pressure high along with corrosion of parts along the way (zincs maintained in the heat exchanger?). Electricity could be involved, I would check the engine block with a zinc probe in the water and a meter for low or no difference in milliVolt when running/not and the alternator putting out some amps. Check that the Battery’s negative connection to the block is good and high power alternator has a Battery negative cable installed (also a standard alternator that is heavily used).
A plumbing expert may help here but is it possible Bronze and Steel are connected with an electrolytic fluid in pipe? The colour suggests copper or bronze in a very reactive state, if this is seawater, where is the bonding wire?
Note: If Fuel goes through pipes, and metal joins couple pipes, all get serially bonded. Likewise any electrolytic carrying pipe or hose requires bonding if metal is used in coupling. Navy has articles about fuel lines.