I am entirely familiar with the social effects of stimulant-induced paranoia, and agree that it an absolute unknown in what reaction to expect from any sort of attempts at contact/communication. Depending on their level of "strung-out"ness you could be seen as anything from some random stranger (zero threat) to some deity arriving to consume their flesh and souls.
however... I do observe that if you see them as an unpredictable potential threat, you're gonna behave awkwardly in regards to their presence. Trying to be careful and not look suspicious just makes one stand out, subconsiously.
If you were to introduce yourself to them at an appropriate time, make some sort of mutual contact/acknowledgement, you may be able to remove any need or presence of anxiety or nervousness towards them. If they know who you are (You said an apartment, you usually know who's living there and who doesn't.) then they will not (in theory, lol) see you as a delusional entity as likely an initial response as any other. Thus removing your need for such anxiety.
If you talk to them, and know they're not gonna trip out on you, then there's no problems, eh?