| It doesn't look serious to me, perhaps as some have suggested mechanical damage of some sort, perhaps the plant equivalent of a bruise (although with the plant it will not heal) or it looks a little like a spot I had on one of my Phals years ago when a drop of water sat on the leaf in the sun and just one little spot burned where the water intensified the rays of the sun. The snail/slug thing sounds possible too. Have you seenany slime trails? If not than probably not slugs.
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