Tile discoloration

[Housewife]

I've had my kitchen floor tiled around October with ufh. Only recently have I noticed some discoloration on the tiles. Is there anything I could do to sort this?

See the image attached the discoloration is only on the edges. I've tried various cleaners from the market but no luck.

It's noticeable to me and I want to sort before it gets worse.

Please help!