Skimming can and does at times crack as your image indicates.
Have you seen any other cracks either externally in the same general area or if this image is of a first floor room are there any cracks in the room below??
it has the appearance of a [so called] hairline plaster crack, if so it is just that? have you tried tapping around the crack to see if the skim is adhering all around the crack?
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