Balding on top? Maybe splitting?

[allygracebz]

I grew this cactus (Craig) from just a pup, and I'm excited that he is now growing some offshoots of his own! But today I noticed a dark spot on top that looks like there should be a spine, but there is not. It is a little dark and then is white in the center as if it darkened and then split a little? The rest of the body is healthy and normal. I want to say he's a rebutia muscula but I don't think I have the 2nd word right. Either way, is this something to be worried about?

IMG_20151126_131555.jpg (21.82 KiB) Viewed 696 times

[esp_imaging]

I'm afraid the phots aren't good enough to help - except that it looks perhaps like Mammillaria elongata rather than a rebutia.
In general, dark spots aren't good news, but without a clear photo it's hard to give more feedback.
If you are in the northern hemisphere, cacti would normally be dormant at the moment. If it's somewhere warm and it's trying to grow it will need a lot of light unless the growth is going to be rather weak and thin (etiolated).