If that was an oil filler cap, it would be “milkshake” like foam from water or water condensation in the oil. I’ve never seen it in a power steering system, but I bet it’s the same thing. Time to flush the PS fluid. Does this car have a liquid cooler for the PS fluid?
Looks like water is in the line. Corrupted the fluids. If you can get it all out and flush it then refill using Lucas PS conditioner. It has worked for me