Guppies are quite willing to nibble some plant or algae based foods, but are not normally destructive or plants, nor eat enough algae to really qualify as any sort of cleaning crew.
Fish poop should be a color that is similar to what they eat. White suggests internal parasites. I would treat them with a worming medicine.
You can do a lot of research about these. Some are specifically packaged for fish, so the labeling will be quite clear about the dosing. Others are sold for dogs and cats, but the active ingredients are safe and effective for fish, too. The trick is finding the right way to administer it, and the dose.
Most internal parasites have a bit of a life cycle, and if you can break that cycle that is good. Many live inside the fish for a while, then come out to find new hosts. Often the dosing of whichever medicine you get will be based on that cycle. Dose for a few days, then stop for several days to a week, then repeat the medication, then a break, then a third dosing.
If you are concerned that the fish may have some other issue going on you can dose with something else, perhaps an antibiotic in between the worming meds. Never at the same time, and do a thorough water change between medications.