What is a little bug that jumps and looks like a flea but is not a flea?

The little brown bugs you describe are probably Collembola, small wingless insects, some of which are about the same size and colouring as fleas, feeding on algae, fungi and debris on the soil surface. They are extremely numerous and widespread, and more commonly known as springtails. The little trigger on their abdomen which they release to escape danger causes them to jump like fleas, but you will need a microscope to see the bodily differences.