Forum:Ancient gear dragon vs alien brain

My friend once attacked my reptile and destroyed it with An ancient gear dragon, can I activate Alien brain considering its after the destruction of the monster?

Also, regarding cards that dont have the "flip" word at the beginning. if i have worm yagan out for example and its set. If my opponent attack and destroys it, does the effect activate? Does it activate if the monster survives?

Thanks In advance.