Robert, it's your responsibility to keep track of this post and any responses you may receive. And you can respond in like to the thread you've started. I doubt that anyone will contact you directly.

The green balls are a brand of controlled release fertilizer (CRF) prills. Some are designed to release their contents over many months...even up to a year.

Let me know if you want me to go into more detail about how CRFs are made and how they work.

First, I have not acquired nor spread any such CRFs, anywhere, especially not around my trash can. Second, the trash can is kept in a secluded place next to the house. So, no one else would have access nor a reason to spread such CFRs on my property. Third, usually CFRs are solid so as to release over time. These are not CFRs. They have an egg-like, thin outer shell and the contents are an egg-like slimy, cloudy liquid. Release of liquid contents can not be time release controlled, it is all or nothing once the covering is breached. Forth, these things are buried and covered as a bunch of about 20-30 in a shallow hole in loose soil/yard debris, like an animal would lay and bury it's eggs. If they are eggs, generally, they should hatch in about 5-8 weeks. I have covered one bunch with a glass jar pressed into the soil to keep them contained if they hatch. I am curious if anyone can identify them at this time.

Slow release prills are called that because they come in different sizes. They are coated with different thicknesses of coatings. Those outer coatings are semi-permeable, containing micropores which allow for the slow absorption of moisture to the inside.

When a prill absorbs its full capacity of water, it bursts open, releasing the fertilizer contents. The thicker the coating, the longer it takes to reach the bursting point.

They start out solid, yes, but don't stay that way. That liquidy center is fertilizer. The brand that I used in my wholesale nursery happened to be that same color; I believe that it was formulated to release fertilizer over the course of four months.

Slow release fertilizers can also be white or yellow (sulfur coated), brown, and probably other colors. Again, they all start out as hard pellets that become filled with liquid fertilizer as they take on water. They're kind of fun to pop!

How the stuff got there is your mystery. Nurseries of all kinds use SRF/CRFs, so do landscapers.