I'm pretty sure there are cliffs with those kinds of decals built into them; I'd swear this was a clear case. If not however, do try the decals FunkyUserName mentioned, cause AFAIK that's the only other option.
What actually happened is that in Heart of the Swarm, they improved the way cliff textures worked. One of blizzard's "improvements" removed the decal on those cliffs.
To get it back you have to go into the data editor and find the Braxis Alpha cliffs that are labeled "OUTDATED" because those are the ones in Wings of Liberty that still have the cliff decals. Besides that change there is literally no difference between the two cliffs that I have been able to notice.