What were they speaking about? Rashi tells us that Moses had divorced his wife:
How did she [Miriam] know that Moses had separated from his wife? R. Nathan says: Miriam was beside Zipporah when Moses was told that Eldad and Medad [newly appointed prophets] were prophesying in the camp. When Zipporah heard this, she said, “Woe to their wives if they are required to prophesy, for they will separate from their wives just as my husband separated from me.”
From this, Miriam knew [about it] and told Aaron.11
In truth, however, it was only Moses who was required to separate from his wife in order to be constantly “on call,” ready to speak to G‑d at any moment. Other prophets remained married and led normal family lives.