The alga that makes the water look green on the top is called planktonic. The green alga will help oxygenate a pond and when it dies off, it can cause a depletion of oxygen in the water and the fish can die as a result. Some strains of this alga can give off a foul smell.
Pond scum is called filamentous algae. It starts growing around the bottom and edges of the pond and can rise to the surface as the pond begins to fill with it. The thread-like alga is greenish in color and attaches to turtles, rocks and logs.
There are two forms of resistant algae that can grow in ponds. The lyngbya alga is a bluish green and the pithophora is a darkish green alga that looks similar to an S.O.S. scrubbing pad. The pithophora grows mainly on the bottom of ponds and sometimes it will appear on the surface of the water, but that is rare. The lyngbya will lie on the bottom of a pond, but can float to the top.
The most common form of algae is the string alga. It looks like stringy hair floating in the water and can become long and slimy to the touch. The alga normally grows by the shallow waters around plants and rocks.
Toxic algae can grow in large ponds and can cause harm to humans, wildlife and pets. The U.S. Department of the Interior Federal Water Pollution Control Administration analyzed medical case histories for a 120-year period for humans who were exposed to toxic algae and it caused throat, eye and nose irritation, muscle pain, fever, diarrhea, vomiting, nausea and skin rashes. Animals that drink the contaminated water died, as noted by the Water Quality Criteria Handbook.