Hi. First, welcome to R2R! :)
I have the same skimmer, in two systems. First, this isn't unusual for a new tank; there's a lot of surface active material in the water to be removed, especially if you used anything like epoxy to attach rocks to each other. There are a few things to try though. Make sure that the air port on the silencer is fully open; if it's closed at all, overflow is more likely. Also pull the water level regulator gate up out of its slot entirely; you can replace and adjust it later as your skimming settles. Depending on the water level in your sump, this still may not be enough. If not, you need to add a platform under the skimmer to raise it up by an inch or two. Again, this can be a temporary thing, as you may wind up removing it later, or fancy adjustable skimmer raising platforms are available. I just use a 10 x 10" square, 1" thick block of acrylic. Then, don't just let the skimmate constantly overflow, even if it's very thin, collect and remove at least some of it.
