Well, I am on a tear about chlorine this morning and this thread caught my attention. There's lots of info on the net about mustard algae.....filter out the "buy our product...it's a miracle" claims and the answer always comes back to.....chlorine....in high enough residual ppm to prevent algae (in any form) from ever getting a foothold.

If it's too late for that, the best answer appears to be, what Ben as always taught, keep the chlorine levels up high enough to combat it and brush it frequently to expose it to more.....you guessed it..chlorine

This seemed relatively informative:

"Mustard Algae" is probably the MOST misdiagnosed form of algae. Mustard algae is a chlorine-resistant form of green algae (yellow-green to brown in color) typically found in sunbelt areas. It often resembles dirt or sand on the bottom or sides of a pool. In our market area (Fairfield county CT), the number of TRUE Mustard Algae cases that we treat can be counted on ONE HAND in most seasons. Mustard Algae has certain characteristics: It can be brushed away very easily, but returns quickly to the same location. Although it usually creates a large Chlorine demand, it has been known to survive in high levels of Chlorine. It is extremely important to remove mustard algae growth from equipment (including the back of underwater lights & ladders).......
Each time I have experienced issues with algae, it is because I allowed my chlorine levels to go below and stay below what I know are acceptable levels.

Test your pools often and NEVER let that chlorine residual get too low. If you already have (let it get too low and algae is present), increase your Cl level to a constant ppm and brush, brush, brush.