A bubblenest maybe? Like this?
If that is it then it means he is happy and willing to breed.
If not then can you take a pic?[/quote]
I spot The Tarantula Keepers Guide! :)
But Op, back on topic, I second the bubble nest suggestion. If its not a bubble nest, I think you should post a photo(s) so that one of the more experienced members could help identify your mystery snot.
This might be ridiculous, but is it possible there is something wrong with the heater and that's where the unidentified snot substance is coming from? If you think that could be it maybe try a different heater and see if it keeps happening. Do you have any other animals in with the betta, like snails or shrimp or something? If so, maybe the substance, whatever it is, is from one of them? If you have them in there with the fish, separate them and see if it keeps happening, if it stops, that probably means the substance is from the snails or shrimp, not your fish. What about live plants? I know nothing about aquatic plants, and especially not their diseases and disorders lol, but maybe it comes from a live plant?