You have a metal corner bead thats had one of its fins covered or taped with a length of paper tape - perhaps both fins have been taped?
This is wrong - no tape is needed its very DIY to do such a thing.
Remove the tape, sand the fin - and fill and trowel smooth with a small tub of Gyproc Joint Filler.
Then lightly sand the finished surface.
It's corner tape that has come away from the plasterboard corner,,rubbish stuff!!
As Roy said, fix a "thin coat corner bead" onto the wall corner, then either pva and skim the whole thing, (best option), or fill and feather out with easifill, (worst option).
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