My habitat space challenge is "satisfy required water area" for all hippos. I thought that I had since the square is green. Do I need 5020.0 m2 or is the challenge bugged?
Yes, you need 5020 square meter.
This is, how the fraction is read: size of the currently available area / required area.
Your "currently available area " is only 3602.2 sqaure meter, which doesn't satisfy the hippos completely. (But they are "OK" with it, so the bar is still green.)