I purchased a home last year that was built in the late 70's. The floor in the entry foyer appears to be slate. It's multi-color blue, gray. My problem is that it seems to be discoloring and getting worse. For example, many areas of the grayish blue tiles are turning a greenish blue. Almost like paint is coming off but I don't think the slate would have been painted. Does anyone have any suggestions on 1. Whether you've seen this before and this is something I can clean off with something, 2. Whether I can/should paint the tile then seal it? 3. Whether I should just clean and seal it as-is because you think the discoloration is not easy to correct.

Thanks a lot for your input.