I did a SLAM about 2 months ago and have been letting chlorine levels taper off. This morning I was doing my daily ph and chlorine test, chlorine was dark yellow and noticed some yellow spots on the corner of the steps. I brushed it and came right off. I then did the DPD test and was at 12. Question is how can I have what appeared to be algea when chlorine level is at 12? Running pump from 6:00 am to 8:00 pm. Help please.