You won't see worms in a fish's feces unless you've killed them with a treatment. Sometimes, with discus that have Capillaria, you can see worms around the anus, otherwise, all that is ever expelled are microscopic tapeworm fragments or eggs.
Metroplex doesn't treat worms - just protozoans and some anaerobic bacteria. You shouldn't dose it for 21 days, I know the instructions say to, but their dose is also way off. The normal treatment length is 3 to 10 days, dose dependent. Metronidazole is very bitter - the old joke is, if your fish are eating the medicated food well, the dose is too low.
Are the fish showing ANY other symptoms?
My guess is that as
@Dragonsreef said, you are seeing mucus. Here is a link to an article I wrote on that (the article is a PDF file on the right of the page):
Mucus: The Slime that Binds Jay Hemdal C. 2020 This article examines the various causes of excess mucus coatings on fish, as well as fecal mucus issues. ------------------- The mucus produced by the fish’s epidermal cells gives them their...
www.reef2reef.com
Jay