Your pond is 0.04 acres essentially a big cement water lily pond. I think a good investment for you would be a 3" dia trash pump (Honda probably most trouble free). Use it to pump out sludge after periodic pond draining every 2-3 yrs.
Since the pond is so small consider a lower cost do-it-yourself aeration system for this pond. This will allow you money in your budget for buying the trash pump and hoses. It will not take a lot of 'heavy duty' big-pond mixing to circulate this small shallow pond to keep it destratified. I will look for the thread form several here that describe a lower cost aerator - see below.
Here is a bunch of homework on RC51's aeration system. He put his in a much larger pond than yours so one similar to it should work well for you. IMO 3/8" tubing should be okay for your small pond if you dont have over 100ft of tubing. Weighted tubing is more expensive but quick and easy for less trouble and more convenience.
http://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=228629#Post228629http://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=227750#Post227750http://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=228882#Post228882http://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=245867#Post245867http://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=232597#Post232597http://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=248108#Post248108Your green water is due to unused nutrients. Many here try hard to get green water like you now have. Maybe add some plants such a water hyacinth if legal in CA. Compost excess plants for mulch and fertilizer. Hyacinth float so they are easy to see and remove.
http://www.ecy.wa.gov/programs/wq/plants/weeds/hyacinth.htmlhttp://www.ecy.wa.gov/programs/wq/plants/weeds/aqua010.htmlWater hyacinth in public water bodies are consider problematic, but they can be beneficial for removing unwanted and excess nutrients form the water thus increasing water clarity. That is what most vascular plants do in water, they remove nutrients for their growth thus purifying the water. Each plant specie has pros and cons.
