White marks on my teeth when I wake up...?

So for the past 6 months i think, I've woken up with white marks on my teeth. When i wake up they are usually slimy and stuff and the white marks go away during the day but even if i brush my teeth they Don't go. Then the next morning, when I wake up, they will be back again. I Don't know if it matters but i wear a standard retainer every night cause i got my braces off a year ago. Can anyone help?