Do any neighbors have septic systems? The algae is worrisome and suggestive
of biologic material rotting. I suspect either a sewer line leak or seepage
yard from an old septic system. I would dig a test hole with a
posthole digger in the greenest part of the yard, to see what kind of mung
seeps into it. Sewage tainted water typically turns the earth around the
area green, especially in sunlight, and has a really pungent smell. Could
also be swamp gases/liquids seeping upwards around your yard.