I use the Lee FCD because I prefer to seat and crimp separately. I've seen some fine brass dust in the finished shell catcher and much more so in my vibratory polisher. I suspect more of it comes from chambering rounds and polishing than from the crimper but in any case I have brass that's been loaded weekly for over 5 years with no degradation attributable to the fcd. If you got a bad one from Lee, why not take advantage of their warranty and ask for a replacement?
Also, I tend to get a tad aggressive with the expander, as I load plated bullets on a progressive moving rather quickly. A generous flare aids in keeping bullets in place during index and aligns them for the seater. The downside is that the mouth may drag on the carbide ring in the bottom of the fcd. This is probably the source of most of the dust. Backing off your expander a little might help.