Every single time I've seen water that was immediately green on fill, it was iron from corroded pipes or tanks. It's easy to remove with time, chlorine and good filters.

Not so easy on Intex type AGs. It will probably be converted into red Iron(II) oxide and form an orange (rust colored) dust sitting on the bottom of the pool each morning.

Now, if it's taken you several days to fill . . . it still could be algae. But, on a vinyl pool algae is almost always accompanied (usually, preceded) by a slimy feel to the vinyl surface (biofilm).

PoolDoc