Re: Higher iron/metals content?
Metal levels up to about 0.3 ppm are alright, especially with a vinyl liner (fiberglass is more sensitive to metals).
Mustard algae can color the water when the FC level is low, but it goes away quickly once you are at shock level. The treatment for mustard algae is chlorine, which I assume you already have. Though really, you don't have any of the symptoms of mustard algae.
Metal Magic alone is sufficient, though some clarifier might help. Once you add the Metal Magic there is some chance the water will cloud up. Clarifier might help with the cloudiness, but it is very hit or miss. Sometimes it helps a lot, usually it does nothing. The cloudiness, if it occurs, will go away even without clarifier, clarifier would just help it go away more quickly (if it works).