What can I do about a potential sexual harassment claim?

If this is the only remark you've made to this co-worker, that has to do anything with her appearance or anything like that, I wouldn't worry.

Sexual harassment claims take two basic forms.  One of these is direct, where the victim is pressured into sexual activity by threats against her job;  the threat doesn't have to be an open one, if the victim's supervisor is the other person involved.  That doesn't apply here.  This, instead, would have to be what's called a "hostile work environment" claim, based on inappropriate sexual remarks (and/or, less commonly, behavior);  while a single very offensive remark can be enough (for example, as in one case, a county sheriff calling one of his African-American officers a "junglebunny"), it usually takes a number of relatively minor remarks like yours to make this fly.