I read that as Niel was meddling in the gallo wars on the side of profaci which is against mob protocal because you're not supposed to go meddling in the affairs of other families unless the boss asks the other family's boss for help. Carlo wanted to stay out of it and dellacroce got involved. gallo humiliated dellacroce in public, dellacroce wants to do something about it and carlo tells him no because he's still trying to stay out of profaci's war.