Is this crack in the mortar joint? the chimney cap? or the brick?

Good question so I got motivated and inspected the chimney again this
morning, here is what I saw. I hope my responders are still with me
here on this thread...

There is a crack around the entire chimney under the top layer of
bricks and under the first line of mortar, the crack is only about,
well, less than 1/8 inch.

The cap, i believe, is the cement sitting on top of all the bricks
which is shaped to drain water? I hope this is correct terminology.
Ok, this cap thing is pretty beat up. after a close inspection I find
that the cap is covered with a cement colored rubbery stuff which is
also starting to let go of the actual cement cap and surrounding
brick, and is getting holes in it from what looks like freezes, at
first i thought moss was growing on it but what i think has happened
is the top was punctured from water freezing and mold is beginning to
grow inside. Anyway I peeked at the cement inside this rubbery
coating and it seems to be crumbling, so I am thinking that i need to
replace that cap with fresh cement.

So now I think my question is, can I handle this repair? I have
replaced cement sidewalks, and have done some small brick wall work.
Also, can this repair wait for the spring 2006 to be completed? Will
the chimney undergo any irreversible damage over this winter here in
columbus, ohio?

Thank you,
Fish