So what? The core principle of Linux RAM management is "free RAM is wasted RAM". The phone will, by design, keep the RAM usage high, such as by keeping frequently or recently used apps dormant in RAM. That way when you want to open them they are already loaded, which saves both time and energy. Conversely closing and reloading them (the effect of using a "RAM booster" app) actually slows the phone down and uses more energy.
If the phone needs the RAM for something else it will clear the space. The best thing you can do is just ignore it and stop worrying. You aren't running Windows 95 here (poor RAM management by Microsoft seems to be the root cause of a lot of misunderstanding about RAM usage).