If he hasn't been doing this before, then it could be possible you have a sick axolotl. Open mouth is consistent with bacterial infection. But sometimes, they are just silly like that.
It can also be a stress indicator too. Check your water parameters, especially your temperature.
Axolotl Open Mouth or Yawning? What Is Gaping and When To Worry? - Unusual Pets Guide
Normally, axolotl open mouth behavior is triggered by stress and discomfort from impaction, improper nutrition, or poor water conditions.www.unusualpetsguide.com
I noticed some months ago... The temperare its ok. 17 degres Celsius