Two kinds of droppings, two different jobs
A guinea pig's digestion runs in two passes. The droppings you sweep up — dry, firm, uniform pellets — are the finished waste. The second kind, called cecal pellets or cecotropes, are soft, shiny, slightly greenish clusters formed in the cecum, a fermentation chamber off the gut. Bacteria there break down fibre the stomach and small intestine could not, and in doing so they manufacture protein and B vitamins. The catch is that this production happens downstream of where those nutrients are normally absorbed — so the guinea pig eats the cecal pellets and sends them through a second time, harvesting on the second pass what the first one made.
What you are actually seeing when he bends down
Cecal pellets are usually taken directly as they are passed, which is why most owners almost never see one. The movement looks like the guinea pig is grooming his belly or reaching for something between his hind legs, and it is over in seconds. Because it is done mouth-to-source, an owner who spots it for the first time often worries something is wrong — it reads as an odd, furtive behaviour until you know it is the digestive system working exactly as designed. Healthy guinea pigs do it daily, often several times, and it is one of the quiet routines of a settled, comfortable pig.
When uneaten cecal pellets are a message
Because they are normally eaten on the spot, cecal pellets found squashed into bedding more than occasionally are worth attention. A guinea pig who cannot reach — because he is carrying too much weight, because his back or joints are stiff with age, or because mouth discomfort changes how he bends — leaves them behind and loses the nutrition they carry. A diet too rich in pellets and treats and too light on hay can also produce sloppy, oversized cecotropes that get left. A dirty, matted rear end alongside left cecal pellets is the combination to act on: book a vet check and mention when you first noticed the change, because that timing helps the vet decide where to look.
Leave the process alone — and keep the diet steady
There is nothing to manage here beyond not interfering. Do not startle him out of it, do not punish it, and do not follow advice to restrict it — quietly preventing cecotrophy starves a guinea pig of nutrients no bowl of supplements reliably replaces. What supports the process is the unglamorous foundation: unlimited hay to feed the fermentation, a measured portion of plain pellets, fresh vegetables, and a healthy weight that keeps him flexible enough to reach. If a habit leaves you torn between normal and early warning, put a follow-up question to KinsLe's Kinny; he answers using the KinsLe Answers library.