It's due to them being ferrous metal and being unpainted/plated. Condensation can form inside the cap because it gets warm and then cools off. Use an old tooth brush with some WD-40 or the like and clean it. Wipe off the dirt and excess spray and repeat this occasionally as necessary.

1970 Plymouth 'Cuda #'s 440-6(block in storage)currently 493" 6 pack, Shaker, 5 speed Passon, 4.10's
1968 Plymouth Barracuda Convertible 408 Magnum EFI with 4 speed automatic overdrive, 3800 stall lock-up converter and 4.30's (closest thing to an automatic 5 speed going)