Mustard algae. I have treated the pool several times with algaecide and shock chlorine only to have it grow right back. My daughter gives private swim lessons and we se our pool for baptisms etc. a lot so I can't have it out of commission so much. Is it true that chlorine doesn't kill mustard algae or that the algae isn't trapped by the filter so I need to keep it dirty? (Pool supply business told me that). I just had it extensively tested and the phosphate is high and the calcium low. Will this likely cure the problem? I am today adding algaecide (I've got to get rid of the poofy slime NOW), phosphate reducer and calcium as well as HCL and extra chlorine. I just spent $60 on the supplies and $40 two weeks ago. I don't want to continue this pattern. I am in the process of reading the beginners information but I'd like an opinion on mustard algae now. By the way, I don't have any problem with green algae at all.