Sludge in Coolant

Doing a 5 year rubber on a client's Tecnam and found this gray sludge throughout the cooling system.  In the expansion tank and deposits in most of the hoses.  First time I've come across this issue.

Borescoped the water pump impeller, but it looked fine with no residue or wear.  Pulled the radiator to have it flushed and plan to flush the water jackets on the heads, perhaps with a solvent like mineral spirits.

Anyone else ever come across this?  Was concerned it might be oil in the coolant, but I didn't see evidence of coolant in the oil when I drained it.  Could this happen if Evans was used and then not properly flushed before changing to DexCool?