Do you mean it always opens up again in RAM? That can happen with certain apps because of the way they're coded. Just because it's open in RAM doesn't necessarily mean it's using any CPU cycles or battery -- if it's simply paused/cached, it doesn't use up any resources except RAM (and if the system needs more RAM, it'll automatically close something else out of RAM to make room).
Open your recent apps and tap the icon for Device Care at the top of the preview. A list of advanced options will popup (App Info, Split screen) at the bottom of the list it will either say "Keep open" or "Stop keeping open". If it says the latter, tap it and that's the issue fixed.