Sunburnt or disease?
 

I shifted this phal to a northern window last week (afraid that it is not getting enough light....) and then an area has turned into light / yellow green, and now it's turning brown.

As you can see the bottom leaf that has been under the shade of top leaf looks normal....

Does this look like sunburnt or disease?

Anyway to help it recover?

I would lean towards thinking it's sunburn but I'm not sure.

Watch for the patch spreading, if it does remove the leaf to stop it spreading to the rest of the plant. Even if it's sunburn the wound from that can allow other nasties a route in sometimes so it's worth keeping an eye on it.

There is also a patch of discolouration on the top leaf too, it didn't show that well on the second photo.

The patch didn't really spread but it turned from yellow-green to brownish colour. Other leaf looks healthy too me.

I have since moved it back facing a SE window.

Looks like sunburn to me as well.

It can happen in a blink of an eye, it feels like.

The good news is, it normally does not kill the plant. It just does not look nice and you might have to cut off a leaf.

It may be a sunburn, but I'd check for spider mites to be sure with a bit of cotton wool and alcohol 70°. Insist underside the leaves. If there's yellow/brown stains, it's not a sunburn.

The fact that there is a clear delieation where the larger leaf created a shadow on the lower one suggests sunburn.

I agree that it looks like sunburn and not even a very bad sunburn. Just a mild one.

Good eyes Ray.

Yes, well spotted Ray!