I do like the idea here. We don't tend to see poisonous Algae in fantasy, even though poisonous Algae is very real. I do like the piece of prose which floats the idea that this is something alien, something that isn't from the wretched world of Mork Borg, as well as a grim picture of what it does to its victims.

The problem is there's not much else. I very much concur with Dungeon Pop's comment, it should be more than just a patch of water that kills you on a failed check. My suggestion would be to have a table of random deliberating symptoms that impact the victim, or worse, spread the Algae, as time runs out for the victim.

Ultimately, I think there's the bones of a good idea here, but it needs some meat. At the moment its just poisonous algae in a world with psychopathic lantern headed men, haunted dolls that can paralyze with an empty stare, and horrors beyond our darkest nightmares.