Inventory system based on mouse coordinates

Hello @ all,

first my english is not the best so please be patient

I want to create a inventory system based on a coordinate system and i need some help.
I know how to check if the mouse is on a specific point at the screen, but how do I highlight this "invicible" rectangle if im hover it with the mouse? I want somethin easy, like a border around for example the coordinates 0,0,50,50.

I hope theres someone how can help me

With best regards:
Kreater