MK3S Belt fraying
Hello all. I just picked up a used MK3S which is my first 3D Printer. I love it and it seems well maintained but I'm curious if I should replace this belt. It prints great and seems to have good tension. I'm always happy to do preventive maintenance if needed but wasn't sure if this was normal. 
I guess the previous owner had an issue with a misaligned idler or collapsed idler bearing, check the idler is OK.
The belt is not at risk of breaking as long as it is not kinking, but the fray might cause tangling/jamming in future so I would trim off the fuzz for now and plan to replace the belt next time I do a major tear-down - maybe not for a couple of years. If the trimmed fuzz reappears then you need to follow the belt path and find the cause.
Cheerio,