autotrophic sand bed

[klas]

I read in Steve Tyrees interesting book about spong/squirt-filtration that a sand bed can become autotrophic (I suppose depending on organic matter build up) and that this can be seen by algae population the sand grains. In my reef I have had no such problem in two years, but recently algae has developed on the sand. I suppose that the build up have reached the limit of heterotrophicity (is that a correct english word?) and the bed has started to leach nutritious compounds.
1. Do you agree with my conclution?
2. What do you recommend? Vacuuming? And if so – how? Removal/change of sandbed? Nothing?
I do not stir the bed very often and there are no large animals doing it either (only what I believe is “spagetti-worms” and “small white snails with long trout”). I have no measurable phosphate and about 3-5 mg/lit NO3. The system has a plenum sand bed in the sump (no light) and is well skimmed. The sand bed in question is in the main display and is 1-6 cm deep.

I hope you are the right person to answer this, or otherwise propose another author or place in the forum for this thread. Thanks!