I'm not sure about the capful size, but 2 ounces weight of MPS per 500 gallons would be equivalent to 6 ppm FC or 13.5 ppm bromine. It may be that locally where that gets added it creates a sufficiently high level of bromine that it temporarily colors the water yellow/brown. So long as it dissipates and isn't permanent it's not a problem -- if it stayed brown then I'd suspect metals like iron in the wtaer.
Normally a bromine floater is not enough to handle bather load. It's designed to maintain the bromine level in between soaks but not to handle oxidizing bather waste. You need to add enough oxidizer after each soak to handle your bather waste. If you do not have an ozonator, then the rough rule-of-thumb is that every person-hour of soaking in a hot (104ºF) spa requires around 3-1/2 teaspoons of Dichlor or 3-1/2 fluid ounces of 8.25% bleach or 7 teaspoons of MPS. You may occasionally need to shock with chlorine, preferably with bleach, on occasion to keep the water clear.
It's true that if you did not establish a bromide bank then if you didn't use bromine tabs for very long (because their spent bromine will add to the bromide bank) then using MPS wouldn't produce enough bromine and MPS alone doesn't oxidize enough of the bather waste.