It's a combination of several different chemicals and minerals. Almost looks like some salt is in there also. Without a lab test, it's impossible to say exactly what it is, but it looks just like the corrosion you would find in a car radiator when the anti-freeze in it wasn't treated with anti-corrosion chemicals and lubricant.
The ONLY cure I can think of would be to flush out your system. Add a radiator flush into the water, run the engine for a while, drain, then do it over and over again until you dissolve as much of it as you can, then put in fresh water, anti-freeze and anti-corrosion chemicals to prevent it from happening again.
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1987 V20 w/1987 150HP Yamaha on a Shoreland'r Trailer
1978 16.5 Airslot w/1996 120HP Force on a Four Winns trailer
1996 V21 w/1993 200HP Mercury on a Shoreline Trailer
All towed by a 5.7L Hemi Durango.
If God didn't have a purpose for us we wouldn't be here, so
Live simply, Love generously, Care deeply, Speak kindly.
(Leave the rest to God) 
Silence, in the face of evil, is itself evil. Not to speak is to speak, not to act is to act. God will not hold us guiltless.
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