— Does the change if you shut down and restart the computer?
— Are you running any anti-virus or so-called "cleaning" apps?
— What does the pane at the bottom of Activity Monitor's window show for "memory pressure" and "swap used?"

Starting with OS 10.9 (~2013) Apple changed the way memory is managed. Today "Memory used" is applying either Windows standards or pre-2013 Apple standards and is no longer the pertinent metric. If Pressure is the the green and Swap is under about one MB, your memory management is doing its job.
The computer that generated that screenshot came is running perfectly.
Also note that any RAM "flushing" required is simply accomplished by a restart. No third-party "RAM cleaners" needed.