The explanation isn't quite necessary - it's embarrassing that he thinks she needs it. She knows what it is... just not really how one ends up with it. Although in the end, his explanation covers both, she still finds a hint of pink on her cheeks.

But even with his warning... she's not sure if it'll happen that way.

"I wonder if it exists here? Mildew, I mean. Lots of things don't, right? Like..." Rinoa pauses and smiles, gesturing toward the couch. "I don't know about you, Professor, but... I haven't really found myself out of breath once since arriving here." And it's not like she's super fit or anything...! It just.. doesn't seem to be a thing. So maybe mildew won't form either? And if it does...

"If there's a problem, I'll get someone to help me throw it out again."