What is this sticking out of my wall?
Given age of building, shape and "magnets stick to it" I'm going with old gas pipe. 

Old gas pipe is not generally something you want to mess with, especially if your use of "tenants" implies rental rather than ownership. Hang a picture so it's covered, or something like that, and forget about it.

If you own rather than rent, and can trace and verify that the gas pipe in question is ACTUALLY, for sure, disconnected, you can remove it, but it's a fair amount of work.