Open mouth is generally a sign of stress however there's really nothing you can do beyond maintaining water quality and the usual nem care tips. I wouldn't feed it till the mouth closes. So far as moving around the tank I wouldn't read into that. It's probably just comfy there. Nice looking sebae!
That's always the most difficult part of keeping nems. It's always exceedingly easy to see when something is wrong but there's almost never any real action you can take.