More of a speculative post than anything in the episode Legacies Delenn and Neroon seem to know each other and have a past of some sort but it is never explained what does anyboady think it was ? Or I am imagining something that isn't there .
Perhaps, with Delenn being a member of the Grey Council and Neroon being a rather well-known and influencial member of the warrior caste, Neroon had some level of interaction with the Grey Council in the administration of the Earth-Minbari War.
Delenn seemed to know Branmer, Neroon's old boss, very well -- well enough to know his final wishes and to decide to act on them. If she and Branmer were old friends, then Branmer probably introduced her to his second in command. They may not have have known each other well, but they at least had met with Branmer before.
B5 is one of the few series with books considered cannon so you can get your answer from To Dream in the City of Sorrows. Neroon is an elder which means he often sits in a governing body somewhat smaller than the US Senate. Just as you would expect a cabinet member/Pres/Vice Pres to know a prominant Senator Neroon and Delenn would have some familiarity.