Well, if they read her memories for Regina and Umber, they would have seen Moss too, wouldn't they?
But then...if they knew Moss was there, why not tell May? Even if they were trying to hide it, you'd think FM would know mentioning the weakness bit would be a dead giveaway.
Unless she only probed the memories for possible people she was sexually attracted to.
Right now I'm going to say they don't know Moss and Maytag know each other.