That is very unfortunate, i have never have any issue with mu CUC eating my macro, apart from my filefish muching on caluerpa.
Red macros seems to have slower groth rate and doesn't go sexual. You could also look at palm tree caluerpa and caluerpa ashmedii as they are also slow growers to save you from frequent trimming.
From my experience caulerpa going sexual never does anything bad to an established reef. When i did a rescape and put a rock on some of my grape caulerpa, most of it eent sexual overnight. And you know what? The tank(15g) didn't even bet an eye... It happened like a few months ago and i found out it died off just a few weeks ago, because I wasn't paying attention to it and gsp covered the rocks the algae was on. I still have some left, but still, going sexual is the strategy only caulerpa using, and if it is not the major algae/coral in the tank and there are no finicky corals everything will be fine. Gracilaria, halimeda, halymenia, chaeto don't go sexual, they either grow, exist, or die. Never was able to kill chaeto tho. Very good technical algae to put in the corners and behind rockwork. Also everything except caulerpa, chaeto and some more stringy algaes doesn't grow that fast. So if it doesn't look like a caulerpa, chaeto or some kind of hair/sponge it won't grow super fast.
I will also say in advance that all this thing about red macros being shade liking is not true. Some of them are, some of them don't. Try and see. But all red macros i had liked strong flow, and I've seen them in the wild - they grow either deep or where the flow and light are the highest. Green are somewhere in between.
Also, even macro safe animals can damage macroalgae severely. For example - some blennies. They don't eat it , but if it's near their hiding place they will eradicate it. My bicolor did this with lot's of species of algae and coral near his tube, before was traded off . He did it only there, the problem is, in a 15 g half a tank is near a tube... My advice, the smaller and more docile the blenny, the less chance it will destroy something near it's hole.