So I guess appropinquare needs the object in dative, but I've read online that ad+accusative is also correct. Ergo, "Noli appropinquare ad deam, Marce!" should also be correct, no?
"Appropinquare ad aliquem" = so, yes.
Both dative, or ad+acc are correct with "appropinquare", but I have no clue if there's a difference in the meaning.
Why is it Marce? And marcus is not acceptable
This happens when you are speaking directly to a person and using their name. This is why Caesar said "Et tu, Brute?" rather than "Et tu, Brutus?"