When I had my 14 gallon biocube I struggled constantly with algae. My glass (or acrylic as I've found out), my rocks, it just never looked clean. (oh how I wish I had discovered the magic that is the magnetic cleaner back then) Yours looks pretty darn good to me so I wouldn't worry, and the rocks will take on a patina eventually. I never knew if my issue was the fact that I was feeding flakes (boo) or that my rocks were super close to the lights, or that I didn't have enough snails.
Personally, I wouldn't run the lights over 7 hours, when I cut mine back a bit I noticed some improvement.
I added another clown too late in my 14 gallon and they never got along. It was pretty bad actually. So, as long as your tests are good, (0 ammonia, 0 nitrites) I'd add another clown quickly. If you want a pair.