My Cockatiel threw up a little white ball (not seed)?

I've owned my cockatiel for about 3 1/2 years now and he is about 6 years old. He was sitting in one of his favorite comfy spots and i heard him move, so I picked him up. His head was covered in a clear, slimy substance, and I found a little ball near where he was sitting that seemed to be covered in the goo too. It is soft inside and hard on the ouside. It almost looks like a little plaque ball. The inside is white, while the outside is a light brownish tan color. Is this normal? Is he sick? Please help. Thanks! :)