Mustard algae is VERY easy to brush off -- but it's apparently hard to remove 100% since it tends to return to the same spots.

I'd just dose with 4 gallons of plain 6% household bleach AND 1 lb of dichlor each evening, till you get the reagent. And, brush your spots daily, preferably right after adding the chlorine.

(I'm sure that somewhere there's a pool store that sells Taylor FAS-DPD reagents, but I've never heard of one that did so.)