Black Markings On Bottom of Feet

[Yellowbug33]

Pool opened 11days 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 twice 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 black algae when I searched for a photo of it online. Pool is crystal clear.
My pool company takes my canister filters at the end of the year and cleans 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.
Can it be the Banana Boat 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 or can it be something from the Polaris robot. I’m just at a loss as I put the Polaris in again to see about cleaning and stopping the black. The kids had fewer black marks on the bottom of their feet yesterday but still some. I posted this under everything else and even though a lot of looks I’ve gotten not one reply in a week. I’d think someone has had this problem. Thanks

 

[zea3]

If it was oily residue from sunblock it would float and form a scum ring like you have seen in the past. Are the kids putting their feet on the sides of the pool around the waterline? Anything on the ladder or the path they walk through to get to the pool? You can always call the pool company and ask about the discoloration on the filter cartridges.

 

[vsyzer]

In my pool where the liner comes up to the bottom of the steps, there's some plastic trim and in a gap between the trim and the steps there's some rubber gasket material that if you touch it you will get black marks from it. Don't know if that's what's happening in yours but I would look for something like that.