Algae growing on your sand in not an indicator that you have adequate light for coral growth. It does however indicate that you possibly have excess nutrients in the tank and that detritus may be building up on your sand bed. Alga can grow in almost any light, from just about completely dark to extremely bright.
A par meter will give you more accurate information as to the amount of light available for good coral growth.
As a side note, lawnmower blennies are pretty good at removing alga from live rock, but they are not really efficient at removing it from the sandbed. There are are several sand sifting gobies that will do a much better job for you.