It looks like those may be some cecotropes? I hope I didn't get my words mixed up... I've had those pop up every once in a long while. I've always monitored for several days before rushing to the vet. Sometimes it may be a one-off occurrence, where the rabbit may have either missed it or they wanted nothing to do with ingesting it because it touched the floor.
But I would cut back on greens for a bit and try to cut it down to just hay, and see what happens.
To me, based on my own experiences with my rabbits... It looks like cecotropes, especially if no other droppings of that consistency have been found since. I would just monitor the droppings and keep an eye out for any more. I don't ever disregard an oddity like that, and if it has me concerned, I'm on the phone with my vet instantly. Don't disregard, but don't worry too much unless it keeps happening or looks to be worsening.
I had my first rabbit, and I panicked over a couple just like the ones pictured. The vet I went to at the time explained cecotropes and how that whole process works... Just so I never panicked like I did again. Back in November, my senior bun- Harriett- had excessive amounts of cecotropes in her pen. I monitored until I could tell it had reached a point of excess. Got her to the vet, and discovered there is some arthritis and may have been preventing her from being able to do what she needed to do. Got some medication, and she was like a new bun after that.