As I've been dealing with this it's crossed my mind that in some ways its as if I started a new cycle on my tank. I say this because I switched out my light bulbs. I had a simple T8 full spectrum then I changed it to a T8 Floramax bulb to help my plants. Then I switched out both of my filter sponges as I do once a month, then of course I added a dose of Excel. This was all done in one day and ever since then I have had this white cloud.
Congrats! I think you have answered your own question. I try to remember that my aquarium operates as a 'system' and changing just one thing can have an affect on several others. I can't tell you how many times what I thought was the 'obvious' cause of a problem was not the cause at all.
Your tank will need to cycle again. Test for ammonia, nitrites, and nitrates and do water changes as necessary to keep levels down. If you have another tank take some mulm from the filter and add it to the filter of this tank to help 'jump start' the process.