Not image processing artifacts in this mosaic, although the mosaic is assembled from 15 overlapping images.

They are fractures in the rock that been filled with minerals. Curiosity is in an impact basin that is filled with sedimentary rocks. JPL explained how they believe the fractures formed and how they filled with minerals in this short informative video LINK

The material in the fractures is sometimes more resistant to erosion by the wind blown sand compared with the softer rock, hence the fracture fill material is slightly higher than the rock in these scenarios