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Hello and welcome to Justanswer!
As far as belt adjustment you have nothing to worry about there. The belt tensioner is spring loaded and automatically adjusting, you'll actually see it moving around a little bit while the engine is running as it takes up belt slack. With the pointing at this mark that tells you as far as stretch the belt is still in very good condition.
The discoloration looks like it may be rust that has transferred from a pulley to the belt. The bare metal back sides of the pulleys can rust as the vehicle sits and it will then be wiped off and transferred to the belt as the engine runs. If the car sat for a while that would definitely explain what you're seeing there. as long as you aren't having any problems with the belt I see no reason to replace it based on the discoloration or stretch. It looks like the back side may be glazed a bit, that would explain squeaking if you are having any.
Thanks for your answer!
Just to clarify with you that my car did not sit for a while as I drive it every day. As far as safety is concern, you mention not to worry about it so I will just leave it as it is.
If you wish to add anything, please let me know.
Have a good night!
As long as you aren't having noise problems from the little bit of glazing then I wouldn't worry about the belt. It looks very good as far as stretch.