Can i choose to play baron nashor on star spring to trigger its move effect to retreat a unit in starspring?

No — Star Spring won't trigger. Baron Nashor's ability is a replacement effect: he enters the Baron Pit instead of the location you played him to, so no non-token unit ever actually gets played to Star Spring (Rule 369.3, FAQ #10250).

The replacement is treated as the entry event not happening at the chosen location (Rule 370.1.a.1), which is why the Pit is what gets contested/conquered and scored, not Star Spring (FAQ #9221, #9728). You could still physically declare Star Spring as his play location, but he lands in the Pit, so no unit is at Star Spring and no move-to-base trigger for you to use. You also couldn't use it to retreat anything, since Star Spring only bounces a unit that's actually at Star Spring.

Caveat: there's no official FAQ for this exact pairing. If you instead read "plays a non-token unit here" as keyed to the declared play location rather than where the unit enters, you'd reach the opposite result — but the replacement-effect rules point against that reading.