It’s a small group of dead pixels on the L-NavCam.

We only really notice them when they image the sky or featureless terrain (as in this post), but they are there all the time when they acquire images in that format with that camera.

Here is a composite of both NavCams that feature that section of the sky in the post drive images from sol 932 https://imgur.com/a/vMCfLxM

Note that the black spec is in exactly the same place on the image.

You can go back in the archive of images and find the same spec on every L-NavCam image acquired in that binning format, it’s just easier to see when there is a plain background.