Summer is coming, the weather is hot and the air humidity is high. After human body perspires, clothes can't be dried quickly, and microorganisms multiply in the perspiring parts, producing protein pigment, which is mildew spot. It's normal for your clothes to have mildew spots on your chest and back where people sweat a lot. As for before, I don't know how long ago this meant. If it had been a few months ago, of course not, because the weather was not hot and the temperature was not enough. If you go any further, you won't know your specific situation.
This mildew spot is easy to handle. From the enlarged picture, you should have the texture of cotton cloth. Just dilute 100 times with 84 disinfectant and warm water and soak for 20 minutes before washing.