App for my iPhone suddenly is in a grey box and it says above it Productivity’s

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Move and organize apps on iPhone

It's a way-to-easy-to-make mistake.

I can assure you that it happens to everyone.

To "undo" what you just "did" :

Simply hold the icon until it starts to wiggle, select "Edit" ...

... and then drag the icon back to the home (or wherever else you want it) ... followed by rearranging as required.

I should have added a bit of an explanation ...

... What I suspect you "did" was to inadvertently "drop" the icon on top of another icon.

What this action does is to create a "subfolder" with multiple apps inside. (in your case a folder named "Productivity")