Well I had several Blue Velvet Nudibranchs in my tank with several BTA's and brown star polyps, but those are not your concern. The problem is that the nudibranchs usually find a way to die. Overflows, heaters, power heads, pumps, filters...etc...
Better to do a check list of why you are winding up with the hair algae problem and try to solve it. ie check your water paramiters, lighting, phosphates and nitrates..etc..
Thomas