Welcome! when i first got mine he would only come out in the evening when nobody was around, he also didn't eat for 3 weeks until i got him PE Mysis (he was quite small, under 2 inches), almost a year later he's now the first one to come out during feeding. they can also go months without eating if stressed.
I had a marine betta many years ago. Beautiful fish. I had success feeding it with seafood counter shrimp skewered on the end of some clear rigid tubing. I'd just hold it in front of the fish and he'd grab it off the tubing. Maybe the clear tubing disappeared under water? Maybe I trained him to eat? Maybe holding the food in front of the fish gave it the opportunity to feed? Maybe all of that or none of that?