Indeed this is a common symptom of allergy.
However, when I spit it out, I find that some of it strangely looks like young candida threads.
In order to address this specifically, you will need to boost your immune system: echinacae or Rei shi mushroom will help. I find it helpful to boost the immune system, although it remains that this is a very basic symptom of candida and it will vanish on its own once you are cured.
I also find that when I do a bowel cleanse, I get more mucus coming out from my throat backdrop. Bowels and sinus are really closely related.