I fought Mustard Algae for years. It is a little, light colored patch that starts to form in shaded and low circulation areas of the pool. It does not form in "lines" like you are seeing. Look at my pool. My pool is on the south side of my house. Mustard algae liked to form on the back long wall at the far end in the deep part of the pool. The other place it would form was in the left corner of the hot tub because of very poor circulation. I had VERY high CYA from using pool tablets.
If you really think it is, I suggest running at the TOP of the FC/CYA level. With 80 CYA, I run a 10 FC because I do not want the MA to come back. I suspect it is not MA.