Unfortunatel this is why most people don't keep macro algaes in their display tanks, unless it's a large tank and they have tangs that will keep it under control.
Your best choices are gonna be to find someone with a larger tank with a fuge, and just trade them your calerpa rock for some bare rock, or build a fuge. and move the rock into it. As for a fuge being strange on a 12g, that's really a matter of opinion, space, and design. My boss had a custom glass fuge made for his 18g half-round tank that is the same lenght and height as the display, sits behind it and actually just looks like part of the tank, the back of the display and the front of the fuge were both painted black so you can't see it from front of the tank at all.
B