He didn't specify what kind?
Most any macro is beneficial unless it is the kind that can bleach out and dump all it's nutrients. You definitely have to watch for that, but I haven't heard of a red macro doing that.
I have all the different types I am in my dt or fuge.
If you want to google the common macros there is;
Gracilaria
Dragons breath
Flame algae
Red ulva sea lettuce
where'd you buy from? if its from pg on central it'sa more than likely gracilaria. tangs go crazy for it. if you have any tangs, drop a piece in and see how they react
It all depends on what it is. Most macros will help with nitrates and phosphates. But, if you're having a problem with either, macros are not the answer. They don't use enough up fast enough. Do you have high phos or trates? It's better to find the issue than to try and put a band aid on it.