La'an still doesn't know what a Romulan looks like after her adventure. The only one she met was surgically altered to look human, although Sera did drop a hint by complaining about the ears. Still, there's plenty of aliens with non-human ears, so not really much to go on.
If she was paying attention, though, La'an did get another clue about Romulan physiology: When she shot Sera, the blood spray was green! Of course, Sera remembered her grandma's old recipe for molecular solvent, so La'an may have thought that was the reason for the coloration.