is the truck running hot? run engine and look in the degas bottle if its bubbling while the truck is running your getting compression gases into the coolant. this will push coolant out when the engine gets hot. the milky stuff on top could just be condensation from heat cycles not sure where you live but if it gets kinda chilly at night it wouldn't surprise me depending on the amount that's up there.
1996 F250 HD 4x4 Single Cab Long Bed, 1997 7.3, auto 3.55s, 6637, E-fuel, CCV mod, 2001 IDM, EBPV delete, Diamond Eye turbo back, high pressure crossover, B&M 14,400 btu trans cooler