I don't think aliens would give symbols the same significance we do. They were very military-like though, that is what I would notice.
I don't buy into a lot of the conventional wisdom on this sort of thing.
Its probably safe to assume that aliens wouldn't be here with our best interests in mind. (Better safe than sorry) Most likely they're an expanding species and they want our planet because it has a reasonably suitable biosphere.
I don't buy into the idea of aliens coming to take our water metals or other resources simply because they all exist in vast abundance throughout the universe. Hydrogen and oxygen are two of the most abundant elements in the universe. Aliens who can travel light years can make water if they need to. In fact an interstellar species can probably make any material they need with Star Trek style replicators.