A common cause is detritus buildup in the rocks and/or sand bed. Low alkalinity, old lights, inadequate flow, and lots of flow all seem to encourage it. Seems GFO-proof. Low-light will slow it. No-light will stop it (and anything else photosynthetic, of course.).
Use a power head or turkey baster on the rocks and swizzle your finger or something around in the sand (e.g.) under flow sources, at bases of rocks. You could see a little pure-white cloud from the sand if it's very clean. Other than that,
nothing is the only acceptable thing to see coming out of your sand or rocks when you test like this.
Assuming you do find detritus in the sand bed (usually the worst offender), then if it's reasonable to gravel-vac the sand bed (I mean deep-clean), you could start with that and see if it's enough. Most of the time, the effort required to un-gunk a sand bed isn't worth it so I recommend a paced removal of the old sand over a few weeks and optionally replacing it or maintaining a bare-bottom tank. And fix the cause! Usually inadequate flow and/or too much rock kinda combine to make things difficult. Sometimes rearranging flow is enough. Other time you need to add more flow. (Sometimes more, smaller pumps are better than fewer, large pumps.)
Let us know what you find!
-Matt
