Pale green cloud at bottom of pond

Observation - Pale green cloud at bottom of pond. Description: Sadly didn't have my camera, but I and two other walkers noticed a pale green cloud in the central deepest part of a very clear pond, I wondered if it could be spawn, any ideas

Sadly didn't have my camera, but I and two other walkers noticed a pale green cloud in the central deepest part of a very clear pond, I wondered if it could be spawn, any ideas