It's normal for breast milk to separate, with a fatty layer on top and a more watery layer underneath.
Some people worry that their top layer isn't thick enough, and that their milk isn't "fatty enough" for their baby.
Rarely, foremilk/hindmilk imbalance (too much foremilk and not enough hindmilk) can be an issue. But you need to look at your baby - not your milk - to know.
If your baby is growing well, is not extremely fussy, and does not have frequent green poop, it is likely not an issue. (However, discuss concerns with your pediatrician.)