yeah that makes sense - i can almost visibly see this happening in my tank. sandbed gets fill of detritus, flow stirs this up landing on and leeching to/from the rocks, feeds the hair algae. i went through a stage of not having to clean my sandbed (other than stirring it up every now and then), it was very clean, white and held no detritus at all (my sandbed 'grain' is large so this was easy to see whether it was clean or not) at the same time my rocks were extremely clean, i had heaps of coralline algae and no hair algae at all. even when i had hair algae, it was only in one place and i had it fully under control in a couple of weeks with rowaphos.
now, my sandbed is dirty (not majorly, but visible) and contains detritus (noticeable as its like small grains of fine, dirty coloured sand) the same detritus is covering some of the rocks especially those in the centre. interestingly the left side and right sides of the tank are near immaculate, the rocks are covered with coralline, there is no "haze" (or very little) detritus and very minimal to no hair algae on the rocks.
the centre however has a film of detritus on the rocks and the worst hair algae too. other than phosphate causing this, the only other differences are 14k bulbs over the left and right sides (where algae is at a minimum) the 10k bulb in the middle is where hair algae is at its worst. additionally, the closed loops and streams provide most flow to the left and right sides, with less flow at the centre/bottom. im sure these are also contributing factors as i would expect to see the same amount of hair algae over all rocks.
i am considering removing the sand and going BB for a little while to see if and how this makes a difference.