Either via the Steam menu at the top, or via your username, click and then choose "change user." That will completely log you out of your account, allowing you to log in to another account.

I was having that issue a few days ago. Closing Idle Master, exiting Steam (the client), restarting Steam, and then restarting Idle Master fixed it for me. Personally I think it was restarting Steam that fixed it, as just restarting Idle Master did nothing.