u/elpitoniso

I got this white joy about 3 weeks ago and everything was going well until it developed a brownish stain on her biggest leaf. I then checked and around it, there was like a grayish color. I started looking other leafs and some of them have very small brown dots and a few have very tiny greyish areas like the ones from the pictures.

Im quite new to plants and have no idea what this is. Does anybody know what they are and how to fix them?