An MBK that's about 5' long is not a young snake, & to me, it appears that he has cataracts in his eyes. They've probably occurred gradually & as long as he's not
fed live (?), it's not going to affect him very much, as he's using his sense of smell & touch for most things anyway. You might find that he misses his f/t prey, or
has some trouble finding the right end to swallow first, but it's not a serious issue IMO. Do ask the vet when he's in for his check up, but from here it doesn't look
like stuck eye-caps, & some people have done horrible damage to their snake's eyes when they incorrectly tried to remove what they thought were stuck eye-caps.
A good snake vet is the safer bet for help with stuck eye-caps anyway, in the event that's I'm wrong about what this is. Eyes are very delicate & you never want to
make something worse. But I'm pretty sure he has cataracts, & not much you can do about it...not a big deal at all, in fact it gives him an interesting look.

BTW, good for you to adopt a rescue snake. MBK's are one of my favorite (king/milk) snakes...I used to have a pair of them years ago, they're beautiful.