It seems there’s a root-level issue; your roots aren't getting enough oxygen, and that's causing the plant to lose vigor. You're definitely giving too much water..
Also, the soil needs to be soft and well-drained; you should use one that's suitable. This one looks very compact, but from the photo, it's hard to tell if it's appropriate or not.
As for the color, nothing to worry about; it can happen in the younger leaves. Good luck, I hope you can fix it!