Simple solution - do NOT use high pressure - do NOT use muriatic acid and absolutely do NOT use the red tip. Using a pump up sprayer, first use a solution of soap & bleach to clean the surface. NO aggressive pressure on the Kool Deck period - 400-500 psi or so is sufficient. Hot water would be better. You can use your soap tip actually (the black one). If you hold the wand a couple of feet up, the white would be okay.
For the rust spots - you can try something like CLR to spot clean with. Or you could give the entire surface an oxalic bath (you cannot spot clean with oxalic acid) making certain to rinse well.
Any questions, phone # is below
Celeste