Hi! I am a new pool owner and we had a professional open the pool and start us off. We were maitaining the chemicals in the pool and at first assumed the brown lines in the pool were sand(not sure from where) and let the robot vacum get them. After a week or two I realized it must be something else and figured it was mustard alge based on the sandy look. We tried just brushing and shocking, but it came back in the same spots the next day. Then we tried yelllow out and shock. And we’ve kept the chlorine levels up for a couple days ans its been growing back in the same spots a but less but in a white or gray color. Our FC level is still very high, but I cannot tell if this change in color means anything, any thoughts??? (I was hoping it meant we killed it, but it keeps coming back)