Guy is nervous about all of this. He has a couple of unique capabilities but he's not inherently dangerous, at least he doesn't think so. So if Cassandra starts getting on his case after he (willingly) sits and talks to her, he may well get very overwhelmed. Rather than hostile though, he'll be incredibly avoidant, up to and including actually physically leaving Skyhold. He wants to help, he really does, but all of this rubs him in absolutely the wrongest of ways. He may well explain this to Cassandra if need be.