Related Clues:

• Birdbath scum
• Foul film
• Dirt on water's surface
• Gunk on a pond
• Red tide cause
• Koi pond scum
• Aquarium bother
• Layer of gunk
• A bird of prey almost picked up fish food?
• Foul layer
• Several seaweeds good in salad uncovered close to shore
• Floating film of muck
• Gunk on an aquarium
• Pool concern