Cromai

“When this attacks” is a triggered effect that triggers when the dragon is considered to have attacked, during the attack step. It does not trigger if the combat chain closes in the layer step of combat. After it triggers, it will still resolve even if the dragon ceases to exist or the combat chain closes.

This triggered effect triggers when Cromai ceases to exist (such as when it flipped back to an invocation) or otherwise leaves the arena.

This triggered effect only triggers once per turn. If Cromai attacks and then leaves the arena later in that turn (e.g. from phatasm), you would still only gain 1 action point in total.

Gaining an action point from the triggered effect is independent of gaining an action point from go again. If Cromai also has go again, you would gain net 1 action point after attacking with it.