That is a very unhealthy platy. If he had been bought, I would have said a wasting disease, but he looks like he's just been born unwell, and is sadly destined for a short life. If you have bought and added new fish since you got him,I'd consider that he might have caught something, but most most likely, he was unlucky.
Fish do a stringy white poop for several reasons. 1) Internal Bacterial Infections causes the fish to stop eating, swell up like a balloon, breath heavily at the surface or near a filter outlet, do stringy white poop, and die within 24-48 hours of showing these symptoms. This cannot normally be...