Is swallowing the small amount of phlegm remaining in the mouth after expelling most of it, along with the saliva during prayer, considered saliva or phlegm, and does it invalidate the prayer?

You should not have intentionally swallowed the phlegm during prayer. The fatwa we adhere to is that prayer becomes null and void by intentionally swallowing it. If you were aware of this ruling, then you must repeat that prayer. However, if you were ignorant of the ruling, then there is nothing upon you, because the prayer of one who is ignorant of a ruling is not invalidated.