Good food for starters are crickets letting them free range in the cage but after you have had it for a bit don’t feed crickets as much, and most of the time you should cup feed some good bugs are dubia roaches and black soldier fly larvae. Neptune the chameleon has a good video on that for YouTube
If the mouth is open it is mostly a sign of stress make sure you aren’t bothering it and leaving it alone it is a defensive sign of a chameleon called gaping