Hammers definitely have a 'mouth'. Most of the time the fully inflated soft tissue does a good job of hiding it. They can do some other funky things like blowing bubbles from time to time. I've had this happen on a number of occasions. The bubble phenom seems to be associated with a branching event where the affected head will split into two at some point after.
I've never seen mine with an open mouth like yours for an extended period though. You also mention soft tissue is deflated. Somewhat reminiscent of an anemone that's either expelling waste, or not doing too well. Only been in your system for a relatively short time, so may still be acclimating. Keep an eye on it & if the mouth remains that way & tissue remains deflated, check your water parameters to see if something's off.