There's a good chance that the fabric dye used in your trousers has failed.
Notice that the discolouration is lighter than the rest of your trousers which is an important characteristic of fading. Stains are always darker than the original colour of the fabric.
Since your situation involves an area that is lighter, it indicates that a combination of wear and care have somehow left your trousers faded. This fits with the location of the discolouration (upper front leg) which is exposed to more wear than the back of your legs, say. It also appears that the tint of the discolouration is a similar to the darker (brown) shade of the vat dyed fabric of the pants.
Therefore, the more you try to remove the discolouration, the lighter it will become. You must do the opposite which is to dye the lighter area to match the darker surrounding area.
On the other hand, faded jeans have become a fashion item; so, maybe as your pants age they will be stylish rather than faulty.
You may want to take the matter to the store where you bought them to discuss the quality of the materials used to make your clothing. There is no assurance that this was NOT supposed to happen. This could be a case of quality control failure.