Coraline Algae fading

I'm very new to sal****er aquariums and I need some help. I have a 10 gallon
aquarium with two clownfish, a couple of hermit crabs and a four astrea snails.
I have flourescent light that I leave on 10-12 hours per day. I have a filter
that hangs on the back of the tank (millenium 1000) and an under gravel filter
- so I think that water movement is pretty good. The water chemistry is as
follows: ph-8.2-8.4, ammonia 0, nitite-0, nitrate - 5 - SG-1.022, Alk -
normal, Ca - 450, dKh - 11. Temperature is kept at 78-80. I also have begun
adding the kent trace elements product. I change approximately 2 gallons every
other week - beginning with RO water. About 4 weeks ago I got some fiji live
rock with reasonably good coverage of purple coralline algae, some sponges, and
a few pollyps of some sort (white translucent). Everything seems to be doing
ok except that the coralline algae is fading in places to a white color with
just little green. I'd rather keep in purple if possible. Any suggestions?