I know it's a little late now, but they make a small, flexible, plastic
trim piece for just that application. Kinda looks like a miniature
version of "J-Channel" that is used on siding applications. I got some
recently at Home Depot for just a few $$$ and used it to terminate
drywall at the fiberglass shower enclosure in my shop .. .. it worked great.

That sai .. another coat of compound will probably fix your situation.
If you want a bit better cure, dig out the cracked stuff, and get some
dry powder compound that you mix with water just before application...
It "cures" rather than "dries" and is much less prone to crack.

wrote:
I'm having trouble finding an answer to this because I keep finding
topics on "fix cracked drywall with joint compound", but I want to fix
cracked joint compound. Thanks for any help.

I took out my bath tub and replaced with a tile shower about 6 months
ago. I put up hardibacker and it didn't quite line-up with the rest
of the existing drywall. It wasn't too bad, I thought, only off by
about 1/8". I applied tape and drywall compound to make up the
difference. Pretty much as soon as it dried, it was cracked all
over. The cracks look like a spider web pattern (pretty much
random). I secured the heck out of the hardibacker so I don't think I
have movement, I think maybe I just applied the drywall compound too
thick, or otherwise incorrectly. It was pre-mixed, so it wasn't that
I mixed it incorrectly.

Anyway, I'm wondering if I can apply another thin layer over the top,
or if it will just continue to crack? Do I need to apply a mesh, or
something? Or do I need to rip out the drywall and start over. No
way I'm ripping out the hardibacker, since that would mean redoing the
tile... I'd do wood paneling, or tile the whole bathroom before I do
that.

O-ya, and if anyone can point me to a web-site (with pictures would be
great) with info on this topic, that would be awsome!