Didymo, an invasive algae, has been confirmed in the Gunpowder river.
This video short describes distribution of Didymo, a description of the algae, and some gear treatment and containment methods for anglers.

The increased time fishing and guiding on the Gunpowder River the past month only solidified my suspicions that something wasn’t right. Nymphing and swinging streamers still produced fish, but the increased frequency of cleaning “slime” off the hook was not normal. Many anglers and guides noted similar masses of slime on the hook and also the river bottom, throughout much of the catch and release areas. Fearing the worst I contacted MD Fisheries and boxed up samples of this strange algae. Days later MD DNR Fisheries Biologists contacted Backwater Angler, placed signs along Gunpowder River accesses, assessed the spread and posted a warning on the DNR freshwater fishing report.