Sirena's race had green skin, and possibly a much longer head than what is normal for a humanoid race, although this is uncertain, as she may have been wearing headgear that was extremely long, which may have ran farther than the length of her head.
Since Sirena was the only member of this race seen, we can only be certain what the females look like, as no male has yet been seen. One possibility is that her troops are the males of her species. It's not too much of a stretch considering that sometimes gender within certain species looks drastically different, as was the case with the Culacaons, whose males were humanoid, yet the females looked like giant snails.
It is also possible that she is the last of her kind. Another possible theory is that her species is not indigenous to Sargasso, and that her kind actually conquered Sargasso and made it part of her interstellar empire.