Yuck! You've got some sort of smelly science experiment involving leaves and "stuff" that's accumulated in the seven months or so since you winterized your swimming pool. If the accumulated gunk on your pool cover is in liquid form, use a cover pump or rent a submersible pump (usually for a 24-hour period) to remove the murky water living on top of the cover.
Consider yourself lucky if you have dried debris on your cover. This can be removed by sweeping it, followed by a quick spray of the hose or pressure washer (really quickly or not at all in drought-stricken regions). The real cleaning will happen after you remove the cover.
Next: Remove the Cover