Black Markings On Feet

[Yellowbug33]

Pool opened 5 days ago. Didn't clear after 2 days so hit it with another 5 gallons of liquid 12.5 chlorine. Chlorine level way high but now normal. Put robot in to clean tiwice and yesterday and today some of the kids have black marks on the bottom of their feet?????
All my readings are fine with the taylor test kit. Don't notice any algae or what I saw they call algae when I searched for a photo of it online.

 

[Yellowbug33]

I just thought of something. My pool company takes me canister filters at the end of the year and clean and store them. This year when they brought them in I noticed a blackish color on the pleats. I'm not sure if they acid wash them or how they clean them but I know other years they look white/yellowish. I don't know why I didn't think to ask the guys why the blackish coloring. Just thought I'd add this as to maybe why the kids are getting the black marks on their feet.
OR can it be the sunblock lotion they are using as I get the black ring in the skimmer and oily after the neighbors kids get out as they all use sunscreen and we don't.