It could certainly be reported, depending on how strict your RP standards are. However, even under the most strict interpretations of our RP realm policy, I don’t see “Sixx” ever having an issue. Yes, it can technically be associated with a band name, but there isn’t any other indication of that. If it were Sixxam, or something similar, perhaps.
There are plenty of words/names that might be part of other real-world things, but usually, by themselves, there isn’t an issue. For RP specifically, we’d look at how it was associated. Sixx can exist on its own and I don’t know if we could say it was instantly recognizable in association with anything else.