Thanks for the replies so far, everyone.
I think the skimmer is producing dry foam, but it's not overflowing past the neck which leads to the collection cup.
Hopefully I won't have any problems with water overflow. My flow is adjusted as per the manual (45 degrees - so half water flow). Until I really put some live rock and inverts in there I won't know how much flow is needed to get the dry foam to the top and over the neck.
Here's a picture of the foam that it's producing with nothing in the tank.
