It appears that your algae is ...
Published by Jeff Lynch, Environmental Water Resource Technician-T-1, D-1
It appears that your algae is growing due to lack of oxygen in the water. A simple fix would be to install a pump hooked to a compressor in the bottom of your pond to get your water circulating. I had a similar situation with a shallow marine and installed a air line to a pump in our marina and it corrected the problem.