You have to report the prize as other income. Coinbase, or whoever paid the prize, might send you a Form 1099-MISC for it. You will report the amount that's shown on the 1099-MISC. If you don't get a 1099-MISC, you report what the value was when you received it. This is ordinary income that you have to pay tax on, just like additional pay.
You also have to report the sale of the Ethereum. Coinbase does not send Form 1099-B, but they will send you a transaction report. The report should show the basis and the sale price. Since you sold it immediately, the basis and sale price will be almost the same, so you will have very little gain or loss, if any. You have to report the sale, even if there is no gain or loss. Since you sold it the same day, the gain or loss is short-term. Any short-term gain is taxed as ordinary income. Any loss will reduce your overall taxable income. But again, the amount of gain or loss, if any, will be very small.
