The amount is taxable in the year received, not the year you moved, if they are different. If you received the payment in 2017, it should have been included on your 2017 Form W-2, and included on your 2017 tax return. If you received the payment in 2018, and it was reported on your 2018 Form W-2, then it should be included on your 2018 tax return. If you reported on your 2017 tax return, the you will need to file an Amended 2017 tax return to avoid being taxed twice. If your company paid you in 2017 but reported it in 2018, and you included on your 2017 return, then they made an error. You would need them to correct your 2017 and 2018 tax returns for it to be properly reported. Or you could file an Amended 2017 return, which is probably easier than getting them to change two years of W-2s.
For information on how to Amend a 2017 tax return, see the following FAQ:
https://ttlc.intuit.com/replies/6603004
