The others nailed it on the head. After 2-3 days you should start seeing a little foam head which will gradually improve over the first few weeks or so. It is the oils and such from the manufacturing process that cause this. In addition the overflow and new sump probably had oils as well which will increase this time while they break down in the system. You will see that when you feed the tank and put your hands in the tank the same thing happens, although recovery is only about an hour or two for that.
So rest assured there is nothing wrong with your skimmer
~300-Gallon in-wall low light fresh water system~
After 10 years, I am taking a break from salt water keeping