I was recently given an 8" cutting of what I think is a San Pedro from pictures on the net (parent plant was 6', six pronounced ribs, blue powdery coating that can be rubbed off to reveal regular green, clusters of short needles erupting from areoles along the ribs). I have it on a covered porch where it gets only a little direct sun but lots of bright indirect light--the parent plant got direct sun from 11:AM to 2:ish. It seems to be rooting nicely tho' a bit thin as I water a bit lightly to avoid overdoing it.

The problem: it has developed a number of small perfectly round black spots. They are mostly between the ribs, some almost the size of a pencil eraser. I tried to scrape one off with a nail but it is just beneath the surface. There's nothing like this the stapelia cutting, 2 sempervivum pups, or moss rose that are in the same pot with the cactus (It's my 'new baby playpen', they'll have separate 'bedrooms' when they get a bit older, LOL). There are about a dozen spots on each side, and they don't at this time seem to be hurting the plant....but I'd rather ask now than regret later: Does anybody know what they might be? Like a virus or some sort of insect damage?