Who Gives A Fish Forums

It's likely fine for your betta, but it'll mess up your water quality in no time. That, in turn, can harm your fish. Live bloodworms could be a better treat option, don't you think?

Stunning betta. Congrats on the new addition.

I'd try using a small paint brush to apply a mix of 1 cup tank water and 6 caps of hydrogen peroxide directly to the affected area. Remove the rock and gently brush the solution over the filaments. I've had success with this method in the past when dealing with similar issues on my daisy polyps.