White areas on my euphorbia ?

It looks like there may have been some type of environmental damage. Possibly cold damage or even sun scald particularly if the plant was moved from partial shade to hot sun. Too much water can also cause die-back.

Unfortunately, once the damage is done, it cannot be reversed. Some folks remove the fan to remove the damage; others simply cut the damaged sections from the plant, but that leaves a misshapen fan. It the damage is not advancing, I would leave it alone.

To prevent the damage in the future, make sure the soil is very dry before watering. When it is not in active growth, probably the winter, water very sparingly. Protect this euphorb from temperatures below 45 degrees F. And when moving from one light level to another, move it very slowly, a hour or so at a time to give the plant to time to adjust.

All of that being said, my best guess is cold damage.