Aeneas' observation of Carthago (where he sees the industrious energic landscape of the City and its dwellers.) culminates in a very known verse: 'O fortunati, quorum iam moenia surgunt!' (Aeneid I.437). It might be noted this is an exclamatory "O" and not a direct speech towards to forunati
Had I to chose a mood, I would go with the subjunctive surgant, pretty much without sparing a second thought, as it seems a book case for the subj. of cause. Like this example Virum simplicem quī nōs nihil cēlet! (Or. 230) (A&G 535.e). Are they not fortunati since they have walls already? Was virgil trying to emphasize something or, more likely, I am missing something. I haven't seen any note about about the mood from the accessible commentators .