Betta attacking tank ornament?

Bettas are fighting fish. My guess is that your betta thought of the octopus as another fish. Try testing it out, put it back in, and see what he does. If he behaves the same way, then he must feel threatened in one way or another. So, I would take it out. If not, maybe your betta felt like playing, and that's all he was doing. Don't worry, I am sure this is nothing serious. Good luck!