Yes, it's a big, terrific beautiful bubblenest.
Some Anabantids do not use material in their bubblenests. For instance, you'll see male bettas kept in restricted, three inch diameter "fish bowls" who make nests similar (but not as nice looking) as yours. Others, like the honey, dwarf and Indian Banded Gourami will utilize dead plant pieces, floating plants etc.
My Paradise tends to use the floating leaves of my Aponogetan as the "foundation" of his nests.
Your betta is obviously pretty happy with the good food he's getting and the care. I find that my Paradise makes these elaborate nests during times of contentment and good feedings.
As for a protein build-up, I really doubt that's what you're seeing. It most certainly is a nest.
K. Blane.