Looks like a bingo. The white stuff was most probably aluminum oxide which is its predominant corrosion product. You're going to have to clean it well, dry it then protect it. Do a search on aluminum corrosion protection. (boats & aircraft) I'd get all the foam out then when reinstalling new foam make sure there is a good heavy continuous layer of plastic sheet between the foam and the hull.
I don't know how your gas tank is set up but mine is a permanent one mounted in the bow. The rubber gas line had been run beneath the floorboard in the foam. I had to replace it when I smelled gas one day while uncovering the boat. What I did was run the new hose thru a cemented PVC pipeline, with 45 deg elbows on the ends, above the floorboard, tight to the bulkhead. (Prevention rather than cure.)
If you have a portable tank i.e. 6 gal metal type, I'd make sure it sits on a piece of wood with painted underside and not directly on the bottom of the boat in case of overfill, leakage etc.
Definitely look at all grounding but also look for any gas leaks/spills etc.
Good luck with it.
Tom B.
(LongLine)