I'm guessing you haven't tested your stabilizer level?*

Mustard algae is resistant to chlorine, but goes away (slowly) when you follow the Best Guess chart 'shock' levels. But, those are determined by stabilizer level.

Order K2006 -- or at least a CYA test that uses liquids and a disappearing dot, and find out what your CYA level is. Once you know what your stabilizer level is, you'll have to set your chlorine level accordingly, or drain and refill, which is pretty wasteful. However, if your stabilizer is above 60ppm, you'll need a DPD-FAS kit to test chlorine levels.

So, the best thing really would be to get a K2006 (boy, we say that a lot!), which has both a CYA test and FAS-DPD testing. (Link in my signature) Do NOT overdo the CYA testing with the K2006 -- it only comes with enough reagent for 6 tests or so. But, you don't need to test more than 1x per month, so that should be fine, if you just are careful.

Meanwhile, if you've been using stabilized chlorine, get a cheap OTO/phenol red kit, and keep your chlorine in the DARK yellow range, till you have a CYA test results

* 'Guess-strips' don't count! They aren't very accurate at best, and they are at their worst when it comes to CYA measurements.