I would definitely say it is activated once per creeping when spawned.
My argument for it is only in the analogy that each biter also uses their ability once per alien when spawned if there are legal targets for them.
As you say, the biters have a "green" backround on their ability, which means that for each biter in the game the ability triggers when spawned and every time it is the alien ability phase. So, if we intepret it the same way for Creepings, it has to be 3 alien moves for each creeping. Am I saying something wrong?