I have a two zone A/C system just installed this summer. One zone upstairs/ one zone downstairs. When I only use one zone the other zone has air coming out of it
, not a lot but enough that the zone not on gets to the temp of the on zone (78). My installer says its supposed to work this way or else the system could grow mold or other bad things. Is this correct?

It is always possible for a zone to bleed a bit when inactive and the damper is closed since dampers always have a bit of leeway in there design.

The mold story is a good one but I cannot confirm or deny it.

Was a bypass damper installed? This routes return air to supply air and is usually modulated by a counter-weight?

Thanks for the answers. My installer and HVAC1000 agree on the bleeding and a by pass damper was installed, so I'll stop fretting.