What is a safe app to clean iPad?

Any cleaning app is like letting your mother clean your room for you. Do you want to face the results?

I.e., None are safe and none are needed. The term "snake oil" comes to mind. They are a solution in search of a problem.

By "cleaning" your iPad, I presume that you intend to fully erase/reset your iPad to its "as new" state - this being straightforward.

To do so, simply follow this process:

https://support.apple.com/109511

Well, there's "cleaning" the way I understand you to mean which is like going around a house, sweeping the floors and tidying up clutter.

Then there's "cleaning" the way LotusPilot puts it which is basically removing everything from the house, putting items in a dumpster or storage unit and leaving you with an empty house.

:-)