Coraline algae help!!!!!!

I have a small sal****er tank (10 gallons) with two clownfish, 4 astea snails,
and 4 dwarf hermit crabs. I put 4 pounds of Fiji live rock into the aquarium
about six weeks ago. The purple color of the coraline algae was a nice
addition to the aquarium, but it seems to be dying. The water chemistry is
good (amonia,ph, calcium, etc. all where the should be). I have one 15w 50/50
bulb (can't seem to find anything stronger for a 10 gallon tank). Any
suggestions on how to encourage the coralline algae growth would be much
appreciated.

Thanks

Bob