In reply to John Kelly:
> For my sins I get to assist in emptying a number of treatment plants, seen plenty turds but they are never green, you sure this isn't some horrible stinky sea gunge, algae could be a contender
^ This. Never seen green sewage, nearly always a very distinctive grey, (plus turds and rags if its from an overflow). Farm slurry maybe? Whatever it is, best to ring SEPA and report it.