Would have to post a picture to be sure. But normally it is diatoms, which are a symptom of new tanks. Now your tank isnt new, but you may be experiencing a mini, or major i suppose, cycle due to you changing substrate. As its likely something like 90% of your tanks 'good' bacteria was in the gravel.......sheer surface area....... and this being removed for sand, which will house no bacteria of this purpose, your tank is probably just adjusting.
Thats my GUESS anyway, but post some pics, and then someone more experienced would most likely be able to give you the definite s.