You're quite correct. Its not a 'proper name', but rather an assumed alias and de-facto name, which still makes it proper to capitalize. Now, if he split the name into its' parts, Actor Mike, it would STILL merit capitalization, because it was in a title.
I was responding to the original post. "Actormike is great" presumes facts not in evidence. In fact, now that I think about it, it's an outright falsehood. Since mike is apparently no longer an actor, he must not have been very "great". Case closed.