Doesn't feel any different than the rest of the tile. Custy said that's where some pool chems sat for a while. But not sure if that is the cause. It is in a high traffic pattern in between the pool and the door. Maybe from wet feet....Does the feel, texture, smell give any clue as to what this may be?
I gotta try the acid. High ph did not touch it.Testing a few cleaners may give you the answer. If it is oils from feet, any good high alkaline cleaner should get it off. If that does not work, I would try an acid side cleaner such as Viper Renew and if that was not the answer then try an oxidizer such as StainZONE or Stain Magic.