Odd. As stated, Al shouldn't "rust" like that. Perhaps its oxidation from your FD clamp (which looks rather corroded) somehow spreading through the paint. (The paint may have a metal base of some sort. Or...oxidizing FD clamp plus clamp digging through paint over the years plus moisture under the paint that's been attacked by the first two factors equals ugly bubbling paint. Only one way to find out for sure. Pry off that FD clamp, and chip the paint and look underneath.