This is a tricky one. The -oid end of a word means 'resembling' or 'like', and implies there was such a thing as a Tharg. Apocopation to Tharg would technical risk causing an ambiguity. Apheresis to Goid may appeal more because it drops the unstressed syllable in the common pronunciation tharGOID.
Personal, i prefer to emphasise the first syllable THARGoid, so I'll avoid any shortening, on the off chance the Thargs make an appearance.
Won't be long before we're all calling them Klaxians anyway. (oh yes they are).