i'm no expert but if you are using granular chlorine as shock you may want to switch to regular store bleach, or pool chlorine which is higher percentage than store. i was having same issue trying to rid of some brown looking dust that appeared to be mustard algae and used pounds and pounds of granular chlorine. did not work. used 4 gallons of 6% walmart clorox for my 16k gal ig pool one evening and by morning stuff had cleared. just anidea. there should be more experienced people posting their thaughts i'm sure.