I recommend doing a throat swab.
It could be a normal mouth infection, I have a similar mild coating on the back of my throat, it started after I had taken some antibiotics for long periods, I did a throat swab, I was then diagnosed with some mouth infection, I don't remember the name, but the doctor said gram negative-something, he also asked me if I had preformed oral sex on anyone before having it.
And try yogurt, it helps a lot
Goodluck!