Sounds like the same thing my Los neck folds gets. Just a combo of sweat, milk, fuzz, etc. Give it a wipe a couple times a day and it will clear up on its own quickly.
I'm typing on my iPhone, sorry.
I'm Melissa, married to my soulmate, S
and mama to my girls:
8 year old K
and 3 year old, A
and my new little BOY!
Most likely nak!