I think the first thing I would do is to get some fine grit steel wool and clean it off, then see if it reaccumulates. In the meanwhile double check all battery connections and wires looking for any line that may be nicked and near metal near the transom. And you may want to run a continuity check between your negative battery post and the metal of the hull to see if there is a common ground somewhere. Electrolysis is often accompanied by the metal pitting.....are you seing that?
I get weird looking stuff on my pontoons on occassion. Sometimes is as you have described. I clean it off, and keep and eye on it and have never had a recurrence. Sometimes, not to minimize it, it might just be some really weird stuff in the water you were at last.



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