Can I play not so fast on a green Janna reaction?

Yes — you can play Not So Fast to counter Janna, Savior's "When you play me" trigger, provided her controller chose one of your units to move to base.

Janna's trigger is a triggered ability (not a spell), and it goes on the Chain as a Closed state. Not So Fast has [Reaction], so you can respond. From your perspective, the unit Janna's ability targets is a "friendly unit," and the ability is an enemy ability — exactly what Not So Fast counters. This works the same way as countering Blitzcrank's play trigger (FAQ #10845).

Two limits: Not So Fast can't target Janna herself (she's a unit and finalizes immediately — only her trigger is on the Chain), and if her controller chooses zero units to move ("up to one"), the ability doesn't choose a friendly unit, so Not So Fast has no legal target.