I have a Aquacraft Nitro Hammer and it runs great except that there is an excessive amount of goo coming out of the exhaust pipe. Does anyone know what that is? The engine is tuned pretty well so i don't think that it's unburnt fuel.

Most likely oil from the fuel, the nitro & alcohol burn off and the oil is the only component left over. I routinely empty & flush the insides of the pipes on my engines at the end of the day.

Seems your engine running too rich. Is your glowplug shiny when you take it out? Close the needle with 1/8 turn in and try again. Before you run the boat again, clean the engine and pipe with wd-40 and after running use only 2 drops of afterrun oil adn a small amount of wd-40. Next time you start the engine both the afterrun and wd-40 will be burned.