If you saw the term “Metal Slug esports players,” it likely came from a misleading article, a joke, or confusion with another SNK franchise like King of Fighters , which does have real esports pros.
There is no known competitive “Metal Slug esports” scene with professional players, teams, or tournaments. Metal Slug is a classic run-and-gun arcade series focused on cooperative play and high-score chasing, not organized esports. If you’re looking for the closest equivalent, some players compete in speedruns or score attacks on leaderboards (e.g., on Speedrun.com), but they aren’t referred to as esports athletes.