Since Hamlet has already deduced what Rosencrantz and Guildenstern are up to, he’s not asking them to disclose their confidential assignment from the king. The phrase “molt no feather” is a reference to falconry and the process of molting.  A hunting falcon will go through a period where it loses some or all of its feathers and is unable to fly until the new feathers grow in. In his metaphor, Hamlet is saying that their secret relationship with Claudius will appear to still function normally, like a bird who has “molt(ed) no feather.”