Been maxing out all my codex scans and this is the last entry I need, I tried letting the scrubbers get infested in the normal non-steel path version but it still spawns eximus. The PvPvE mode also seems to only spawn the eximus Babau.
I am actually not sure if a none Eximus Babau can spawn outside of the quest.
You can completed the Codex entry by scanning the Spectral ally the Ballistica Prime's unique trait spawns when killing a Babau Eximus with a charged shot.