Then hes just blowing a bubble nest. There is nothing to worry about and most betas build one at some point in there life.
The nest last about a week unless you remove it, and how long the betta will keep doing it is up to the betta. Some build one and that is it, or they will doing it a year or more. The males build the nests to put eggs in, then the betta fry hatch in it.
I've heard from my friend that can contain bacteria so I usually removed it after a couple days.
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