Adjectives answer the question which or what kind for a noun.
But you can also answer these questions with in a wordier form with a {noun} that is {property} pattern, since to be is a copular verb that completes with a subject complement, and adjectives work as subject complements.
I looked at the car that is red.
I looked at the red car.
No difference in meaning, it's just a stylistic choice of whether you want to use more or less words.
Typically you want to use the adjective unless the adjective is something like a complex adjective phrase or clause, or you want to use more words to ensure a listener/reader understands better.
English likes to place adjectives before the noun, but it's possible to for them to be postpositive or happen after the noun. The Wikipedia article on these details when this usually happens. In this case, I believe the following applies:
One common situation in which adjectives appear postpositively in English is when they qualify compound indefinite pronouns: something, anyone, nobody, somewhere, etc. Examples: We need someone strong; Going anywhere nice?; Nothing important happened.
Even though only thing is not a pronoun, it's working enough like one possibly for the above to apply.