It's safe to clear apps from the recent apps menu. But, it is suggested that we leave it there and not clear it because the Android OS is smart enough to identify which apps you are using and done using. Also, launching apps will requires more battery power than the ones who are found and lauched in the recent apps menu. By this, it will shorten (but not too significantly) the battery life of your device. Also, clearing RAM too often may cause some issues.