I fear I may have mustard algae. I have this spot on the floor where it meets the wall on the side of the pool that gets afternoon shade (west). It is yellowish green. The first time I noticed it I brushed/vacuumed etc. and it went away. Now approximately a week or two later it is back in two spots on the same area of floor. My chlorine has remained between 4.5-7.5 (varied because trying to adjust SWG)since opening this season. After the first time I noticed this spot I did OCLT and have never lost even 1 ppm chlorine overnight. Last OCLT was 0 loss (last weekend). Water has been clear all season (still is) Could this mean Mustard algae? If it is I know I will need to raise chlorine to mustard algae shock level. I've never used any chemicals besides salt/MA/CYA/Borax since the pool was installed and initial treatment by builder in 2011. It seems like this would be ALOT of bleach to raise to mustard algae shock level so my question is if I should use cal-hypo instead since CYA is good(I have no experience with these pool chemicals). I have only tested CH once since it is a vinyl lined pool and I tested 30 (I will double check this).
Todays numbers (tf-100)
PH-7.3/7.4 (hard to tell exactly)
CYA-70(ish...cloudy day but consistently dot has disappeared between 60-75 since 5/24/2013)
CH-30 (5/27/2013 will check this again)
I know everything that I know about pool care from this site so will follow advice given (sorry if I left out any detail). Found this site shortly after install in 2011 and have never had a pool store test my water or sell me chemicals........but now my worst fear is mustard algae All advice welcome.