it's time-release fertilizer. it's usually lasts for 2-3 mo. sometimes they are green or red.

What kind of plant is that? The things I see in your soil look like part of the roots. Some plants, like asparagus fern, form tuber-like growths. Time release fertilizer would be in balls smaller than a BB.

Yeah, looks like tubers. I recently repotted this store-bought Calathea Makoyana, and the roots looked very similar.