What does よろしく mean when someone uses it as a greeting but I already know them?
よろしく as a greeting for someone you have not met is the 'exception' to the rule. Most often, it is a hard-to-perfectly-translate word used for giving someone regards. (most literally meaning "take care of me")
In the case of using it as a greeting for a new acquaintance, I believe it's frequently seen as
よろしくお願いします, since you generally need standard formality with strangers.
よろしく alone already indicates a level of casualness/informality between you and the speaker, and most likely means it's being used in a more general sense of please/thank you.
With highly codified expressions like this, it really depends on the context it was said in. It might be helpful to look at some example sentences online, and see which of those examples best fits the context you heard the word in. Here's a start? Tofugu has a lot of good resources/explanations as well for Japanese language topics.