What do you mean by greyed?
Sequencing of external MIDI CCs works via the sequencer mod lanes, not sure about mod matrix destinations.
But it's barely useful in practice, because there is only a handful of preselected CCs that can be sequenced.
So for example I have Quadrantid Swarm nearby, that has MIDI implementation of only CC1 - CC7, but PRO3 can sequence only CC1, CC2, CC4 or something like that, so I can't sequence the controller I would like to.
This is totally unnecessary limitation on my opinion.