I think Jordan's got the right idea. My first thought while reading this was "sounds like a lack of oxygen". Do you have the resources to get a
solar fountain or aerator that will help reoxygenate the pond? Algae bloom is often associated with lack of oxygen.
As an immediate and temporary fix, can you encourage visitors to the pond to sit on the wall and kick their feet in the water? This will help aerate the water and kids will probably love it.
Then I would get some plants in, whatever native reeds grow in your area, get those and put them in pots in the shallow end - that will help filter out elements that have become toxic.
Do the fish and turtles have enough other food to eat besides what people are giving them?? If so, I would stop people feeding them extra food for a time until the pond comes back into balance. After that, I would consider putting up signs indicating that over feeding is polluting the pool and to respect the health of the animals, please don't feed them. (or have only a certain amount of food that can be put in the pond per day).
Do you have some pictures you can post here?
I personally feel that oxygenation will solve many of your immediate problems and that moving the animals and "fixing" the pond might do more harm than good. Hoping some others weigh in here as well.