Green water-are buckets I set out in the sun and add hay, grass clippings, oak leaf to grow all different kinds of micro-critters for fry food.
I collect mosquito larva with a small meshed net and place them in fresh clean water, I pick out the non-living stuff and run the larva through the net again and rinse lightly under running water. The really tiny larva fall through the net and I use an eye dropper to feed these to my older fry I have in the house and I dump the larva I have in the nets into my other tanks and watch the feeding frenzy.
If the mosquito larva are alive/living in the bucket of water they should be safe to feed your fish, I would not add any of the mosquito larva water to the fish tank.
I don't know of safety issues with maple trees, but if the mosquito larva are alive it must not be too toxic.
Black walnut tree-all parts in large volumes have been known to be toxic to aquatic life, especially the nut hulls.