First of all, I fail to see how you could check how all the processes running are legitimate, even if the malicious process did show up in task manager. The list of "usual" processes would change every time you install any new software and might even change after some software updates itself. Additionally, you might have a hard time telling which processes are "usual", since any malware that has established persistence on your device would show every time you reboot, and you might mistakenly start considering it "usual" too.
Even if you could do sort out the trusted processes, any malware that leverages something as simple as "living off the land" techniques (which is basically just using preinstalled binaries like powershell, cmd.exe, mshta.exe etc to carry out malicious actions) would completely bypass your test.
Of course, there are more sophisticated techniques like which MechMK1 and Dan Landberg refer to but its actually really simple to defeat your (quite impractical) check.