I'm barely an amateur when it come to even thinking about turbidity/clarity/blooms, but my guess is that you dropped out alot of "stuff" which improved the clarity and an existing algae has taken advantage of the new light penetrating the water column and has given you that nice green color. Shorty hit on this in his post.
At any rate, it looks much better green than brown so long as it does not turn in to thick pea soup. Keep an eye on it and post some clarity data as you go. Do you use a secchi disk?...very handy and easy to make.
I had the peas soup happen the first year and the second, but not this year. Mine were not associated with a alum treatment, however. Here's my thread on the first time it happened...
http://forums.pondboss.com/ubbthreads.php?ubb=showflat&Main=35937&Number=470475#Post470475
