Looks like you're mixing water & exhaust and getting a nice froth! Might be coming from a leaking exhaust manifold baffle plate, or leaky manifold gaskets. If there's enough room, you can try pulling the exhaust plate in place, without pulling the powerhead. If you find a lot of "milkshake" inside the exhaust area, odds are that's your leak.
Another possibility although very slight is a leaking powerhead base gasket, spraying water into the exhaust. But much more likely to be problems in the exhaust manifold area.
HTH..........ed
