Mos the bearded dragon certainly seems to be enjoying his facial massage. Notes Pauly, "I wouldn't interpret this gape as a submissive gesture. It is really challenging to understand behaviors in an environment so far removed from what they would experience in nature (a giant mammal giving them a massage). My guess is that the lizard likes having the region around its mouth massaged and opening the mouth just directs the massage to the right area." While Mos loves this head massage, your lizard companions may not - find out what works for you both to keep your lizards feeling safe and happy.
Some final words of wisdom from Pauly: "There is not a lizard equivalent to a dog's tail wag or a cat's purr." Since lizards are prey for so many animals, Pauly adds that their responses tend to focus on defensive strategies, so, "the most widespread 'positive behavior' would simply be that a lizard appears calm. If an animal is resting, not breathing rapidly, and not engaging in any defensive behaviors then this is all positive."