Does it look like this?
If so, that is a bubble nest. Bettas make them when they are happy with their environment and is also a sign of being healthy. Bubblenests are what attracts the female to help with breeding. The bubblenest is where the male betta places the eggs once fertilized and will care for them until they hatch.
So, if that is similiar to what you're talking about, it's a very good thing!