Hi Joe,

You may have mustard algae or perhaps some other species of algae growing in the pool. Mustard algae is more common in southern states. There are two ways I can think of to find out.  One is to get a water sample and have someone look at it under a compound microscope.  You'd probably have to call a local high school or college biology department and ask if someone might look at the sample.  You really just want to know if it's an algal species, so the person looking would not need to be a specialist in algal identification. If you're in Oakland county, you might first try contacting Oakland University's biology department (<personal data hidden>). I don't know if people are on campus right now (at MSU, we are not). But, if they are, make sure you bring them (or mail) a fresh cold sample in a plastic or unbreakable bottle. If you get confirmation, then you could use an algicide or a shock chlorine treatment on it. Your local pool store should have products for this. The other way is to go ahead with the treatment and see if it solves the problem.  I'm not advocating this approach however since it may not be an algal species, and you'd be spending money on the product without getting any benefit from it.  Thanks for the question.