Film on top of water

Hey guys,



So I set up a 20 gallon long with 24 pounds live rock and 16 pounds live sand and after I added my water it has had this film or dirty look at the top of the water. I don't know if it is dust/dirt/other things…



Have you seen this and what should I do to fix it?



also, should I set my power heads facing each other on each end of the tank? I have a 240 and a 180..



Thanks!