The red “x” is usually an indicator of palm rejection, but can randomly occur sometimes because of the stylus.
On your iPad, try turning off and back on bluetooth. Sometimes that restarts connection.
Also, try turning off bluetooth on your Mac, if it is enabled. Sometimes the signal conflicts.
Re-syncing the stylus can help reestablish connection.
On your Mac, go to Astropad’s Preferences, and click the “Stylus” tab.
At the bottom of the window, click “Remove stylus”. Then try pairing on your iPad again.
And this stylus was pair in Astropad under the Adonit logo?
Let us know if the red “x” still persist after trying this.