Looks like some minor rust or it's just starting to rust. You could strip out the rust with phos acid. It will not hurt the paint if you spill some on the tank as long as you wipe it off. It will eat the rust without eating the base metal. Then you'll probably want to line the tank with POR-15 or Caswell's liners. PITA process, but can be done without damaging the paint if you are careful. Watch the POR 15 prep products. The Marine Clean will attack your paint.