Hisoka is recognized by the boss as a Spider, the only way I could see him not being a Spider is if it's really someone using Nen to impersonate the clown. That doesn't seem to be the case as the clown has acted exactly the same whether or not he was around the Troupe. Even his inner horny thoughts are the same.
I don't think the fake tattoo means anything. Maybe Hisoka is Jewish.