Its the "mucousy" stuff in their stomachs, athat is brought up into their crop to help digest the food that will be brought into the sotmach and elsewhere. Its normal. But if your are worried, an avain vet check is in order, and they'll to a throat swabbing.
When I got got Bokka, they did a throat swobbing and it proved he had an over abundance of bacteria in his crop (and we did other tests during the vet exam and he had a huge list of issues). But he still does the "spit" thing, so thats normal.