[Delphinus]

Most definitely razor caulerpa.

Beyond caulerpa some other cool algaes are Halimeda (which is calcareous) and sargassum (or "kelp").

I'm not an opponent of keeping caulerpas (or other macros) but it does require careful management. Ideally you want the stuff in a lit sump or a refugium of sorts where it doesn't matter if the stuff totally takes over. In the main display tank there will likely come a day where you don't really care for it up there, and sometimes you may find it may have been better to never let it get up there in the first place, rather than try to remove it after the fact. But that's not say we should never keep any: that's up to the individual tank keeper. If you're having fun then have fun with it.

But be aware there are downsides. 1) it does have a total weed mentality 2) sporulation can be hard on a tank if it happens often or if it happens in a big way 3) macros are known to have allelopathic behaviours (which is a case of being able to produce chemicals which interfere with or inhibit the growth of other species). So even if you choose to keep it, being strict about regular pruning would probably be quite prudent.