Sometimes the top or side of a tomato develops a whitish or yellowish plaque on the sunniest side of the plant. Over time it can become flattened, papery, and grayish white.
The fruit is simply suffering from sunburn or, to use the correct term, sunscald. It occurs when a fruit that was previously shaded is suddenly exposed to full sun. This happens when the upper leaves of the plant are removed or, in the case of plants grown in pots, you have changed the plant’s orientation.
Sunscalded fruits will not heal, but do remain edible. Just cut off the damaged part.
To prevent sunscald, avoid removing the top leaves of your tomatoes and if you need to move a potted tomato plant to a new spot, either do it on a cloudy day (thus giving the fruits a chance to adapt to the change) or keep its original orientation.
Sunscald can also affect other fruit-bearing vegetables like peppers, eggplants, and cucumbers and the same preventative treatment applies: just don’t suddenly expose them to full sun!