These steps will show you 3 potential ways on how to hide what you are playing. This will work with any game in your Steam library.
Three Ways to Hide What You Are Playing
Steam “Offline Mode”
In the Steam client on the top left, click “Steam” > “Go Offline…”
This will disconnect Steam from the internet, allowing you to play without anyone else knowing you are playing. (This will restart Steam)
To return back to normal, follow the same steps but click “Go Online…”
Sign out of “Friends & Chat”
In the Steam client on the bottom right, click “Friends & Chat”, from their click the ▾ next to your name in the “Friends & Chat” popup window, then click “Offline” to sign out of “Friends & Chat”.
This allows no one else to see what you are playing while still being online on Steam
Change name of the game you are playing
In the Steam client on the bottom left, click “ADD A GAME” > “Add a Non-Steam game…” > go to the Steam directory of the game and select the .exe.
(eg. C:\Program Files (x86)\Steam\steamapps\common\iVIBRATE Ultimate Edition\iVIBRATE Ultimate Edition.exe)
This will re-add the game to your Steam library, making two entries in your games library. From there you can right click on the game in your library, go to properties and change the name of the game as well as the icon. This allows you to make it look like you are playing a completely different game. This will also affect Discord if you have Discord showing what game you are currently playing.