It's "alien" in the meaning that it is weird and unlike what is known from before, it is NOT alien in the meaning that its origin is not Earth.
I am still waiting to read the full research paper on this. Does it contain phosporous at all or is just the phosphorous in some molecules exchanged with arsenic? If all phosphorous atoms are exchanged with arsenic atoms then these organism may very well be a relic from a time when living cells on Earth were recent, because this would imply that it's DNA is different from the DNA of all other known organisms.
EDIT: I just read the first post and it really looks like the phosphate groups in its nucleic acids (DNA and RNA) are exchanged with AsO4-groups (?).
EDIT 2: And by reading a little bit on the Net I find that only parts of its phosphorous are exchanged with arsenic.
Still, very interesting data for us interested in astrobiology and the evolution of life on Earth.