If it's only in your fuge, let it be! It will keep the algae out of your tank AND it will keep nitrates and phosphates out of the water.
I bet your water tests Nitrates = 0, Phosphates = 0. Thats what algae does. In actuality though, you probably have a fairly high nitrate and/or phosphates but the algae feeds off of it so your tests show zero.
Another thing could be your bulb over your fuge. How old is it? And a 400w MH over a fuge seems like WAY too much light. But I'd be afraid to change it to something else because then the algae might migrate to your DT.