Answer:

It sounds like a circulation or filtration problem. You might have a "dead spot" where there is little or no circulation occurring and therefore no chlorine getting to the area. In this case, the algae usually starts in the same place over and over. You should be able to fix this with the direction of your return jet(s). Or you may not be getting enough pressure from the pump/filter to properly circulate. Or it could be that there is algae resident in your filter or plumbing somewhere that is allowing it to recur. Clean and/or backwash the filter and make sure you are getting the proper pressure.