If I am in Alter-Ego while I have the warriors of the great web card attached and a web warrior ally leaves…

Yes, Warrior of the Great Web’s Response can be triggered if a Web-Warrior ally leaves play while you’re in Alter-Ego form. That said, your Alter-Ego can’t gain or use a basic power type it doesn’t have; but, with WotGW creating a lasting effect giving “attached character” +1 ATK, you could flip to Hero form and gain +1 ATK until the end of the phase.