My dog spits up white slimey mucus and spit with some digested food. Had her at the vet and they said she looks good. What could this be?

How often does this happen?

What are you currently feeding?

She and my other Whesten are getting Flint River Ranch Organic Trout and Sweet Potato food, with no wheat, corn or any other additives. Wheaties are very prone to PLN, which is a protein losing disease for that reason they have their blood tested twice a year. She is also on a probiotic, well I realized that I ran out and didn't give her the probiotic for a couple of days, so I am thinking that may be it because she was doing this before I started her on the probiotic. Everything from the vet came back normal, so I am guessing this maybe the problem.

It's definitely possible, if they are used to have the probiotic then missing a few days could certainly throw their stomach out of whack.

I asked about the food for several reasons, you have answered one, but another I was thinking of is that quite a few dogs have trouble with dry food in general. My mother has a Basset Hound who vomits after he eats plain dry, it has to be soaked as he swallows the pieces whole and they end up being brought back up partially digested.
If it has only been happening recently then I doubt it would be that.