Greenify's mane function is to hibernate any installed apps. This allows users to to quell overly active apps that are draining their battery without having to delete them. This can be done with or without ROOT.
The Greenify (Donation Package) has a few extra experimental features that the free version does not offer. They require ROOT and the Xposed module to be installed. Using these features can help with the compatibility of the rest of the Greenify app, which is a big plus.
When using Greenify without ROOT access to hibernate apps, it has to be done manually each time. There is a widget and shortcut that users can set to their home screen to make this action a bit easier though.
Some screens are too bright for night-time use, even at minimum brightness. Because Pixoff turns off pixels, you can effectively reduce screen brightness below the minimum amount.
There is a configurable option to separately and fully put the wallpaper to pure black in order to help save battery life. This is configurable depending on the level of battery.
For those that use multiple devices there is a cloud sync feature that allows the saving of settings and custom profiles. This way there is no need for setup on a new or different device.
Because it overlays a very thin black grid, it is effectively reducing the resolution of the display...however the pixels won't be spaced out properly (for example instead of having 8 pixels evenly spread out, now you could have 4 regular pixels followed by a black pixel, followed by 4 more pixels and another black pixel. This will distort the image.
There are 5 levels to choose from - Mesh 1 is the lowest setting, and Mesh 5 is the most aggressive. You can choose which level to run at to best compromise between battery life and picture quality.
Pixel OFF basically overlays a black grid over the existing picture to prevent some pixels from lighting up at all. This reduces the overall power draw from the display
Clean Master and similar apps were popular when Android still had memory problems (more than 2 years ago). Since then, virtually everything Clean Master does has been added into the base Android code. You can clear RAM through the settings on your phone if you feel the need, however Android is now smart enough to remove apps from RAM when needed (when more RAM is required by other more recently used apps, or when the background apps are taking up too many resources).
If anything, Clean Master takes more battery because it's constantly scanning, and any app it removes from RAM will have to be re-loaded in the future which defeats the purpose of more RAM (not to mention re-starting an app taxes the processor, draining battery faster). As the saying goes, "free RAM is wasted RAM".