Mustard algae options:

1. Run chlorine levels at the 'algae' or 'shock' level continuously -- see http://pool9.net/cl-cya/ -- and brush a lot.

2. Run normal chlorine levels + maintain phosphate levels below 125 ppb (0.125 ppm).

3. Use lots and lots of copper, turn everyone's hair green, and stain the pool.

I'd recommend #1 first, especially if you've never had mustard algae before. If your pool is not one of the not-so-lucky ones prone to mustard algae, it may stay gone after you've cleaned it up.

But if you find you're struggling to keep it away, we can look at option #2. However this requires managing phosphate additions to your pool from multiple potential sources, including the fill water.