I get that too... on all my betta.
Its actually, I've found, from the water conditioner. After about 3-4 days, it clears up on its own, when I add the conditioner, it comes with a vengence. Its the scale coating slime that comes with my type of conditioner. Its more of an aethetics issue than an actual real one, in my opinion, as the fish end up slimy and happy and their scales are nice and protected. I think it may come from adding a little too much when doing a waterchange. I know it is highly altered by the amounts of the conditioner I add to the tank. If I have a situation where I have fin tearing, and I add the slime coat doseage to help it heal and protect it, I get the slime on the top of the tank.
Thats all I've noticed from my observations. (I use Aqua Plus.)