You may move +1/+1 counters from any number of other permanents you control, not just from one.

You choose which counters to move as Aetherborn Marauder's triggered ability resolves.

You may choose to move zero +1/+1 counters.

If a creature has been dealt damage, that damage remains marked on it until the cleanup step. If Aetherborn Marauder takes counters from a creature that has been dealt damage, that creature will be destroyed if the damage is now lethal.