If you drained to a foot, then refilled and your CYA is still 90, I would have just drained again to get the CYA to a more manageable level, such as somewhere less than 60. It's not very practical for continued maintenance and for the SLAM procedure to do so at that high stabilizer level. A picture of the water would certainly help.
Bromine treatments can produce results because bromine is not subject to the stabilizer like chlorine is. However, once you put bromine in your pool, it's a bromine pool until you completely drain and refill. That means your sanitizer, now bromine, has no protection from sunlight like CYA does for chlorine. Even if you add chlorine to a bromine pool, it instantly converts oxidized bromide (which remains in your pool after adding bromine) back into bromine so you still only have bromine in your pool. Long term, you'll have more difficulty maintaining sanitizer because there is no CYA analog for bromine.
Mustard algae is really no different than any other kind as far as how you treat the problem. It's only at the end of the SLAM procedure where you do something a little extra for suspected mustard algae. Mustard algae does exist but it's not the big boogeyman that is sometimes used by pool stores to upsell products or ill-advised bromine treatments. Reducing your CYA and then proceding with the SLAM fixes the problem long term rather than a band aid that will cause another problem.