I received a 1" tall seedling that I potted about a month ago. It's my first foray into cacti. Seems to be growing now. However, I've noticed there is fuzzy gray mold growing around the pot near the top of the soil. And it also looks like there's some traces of it on some of the perlite in the soil. Should I be concerned about this?

Is the plant in high humidity?

No, it's winter here so the air is quite dry.

Can you find the herb pau'd arco in your area? Make a dilute tea and apply it with a spray bottle. If you can't get that, chamomile also works.

If the plant is in low humidity then you have nothing to worry about ime.

Mold all seems to have went away now that I started top watering, but the cactus doesn't seem to have grown at all since I last posted. Wondering if I used to much perlite in the soil for such a small cactus. I did the mix recommended for adult cacti. Hate to transplant again so quick though. Also wondering if it could use more light.

Since it was uprooted and shipped to you just recently then that's probably why it's not growing very much. In my experience transplanted seedlings stall anywhere from 1 week to 3-4 months. They need to get established in their new home before they start growing. Transplanting it again is a bad idea. You can always just supplement nutrients via fertilizing if you feel there isn't enough in the soil.

Yes, leave her in the ground! Cacti require patience. Just trust that it will figure it out. Sometimes, after planting a cutting, a new column will sprout out of the ground even before I see much noticeable growth on the main cutting. Like sometimes it seems to abandon ship and feed the resources to a new arm. Whether its going to do this or keep feeding and growing the established arm, a month is pretty quick to be seeing any substantial growth. Once they get movin, they move though. Good luck!

*le sigh* I came home to find the little guy laying on its side in the pot looking like someone dropped something on it. Figured I might as well re-do the soil a little bit around it and when I went to water it, it turned out the soil I put in doesn't seem to drain at all...
So wet and compact in there right now feel like I need to let it dry out a bit before digging it out. Can't leave it in soil that forms puddles on the top can I? Gonna be surprised if this thing ever gets around to growing now. I guess they're kind of tough though, eh? We'll see.

If you must replant it then let it dry out for 2 weeks or till the soil is completely dry. carefully take out all the soil in the pot then carefully clean the soil off the roots best you can. Put it in it's new home and don't water for another 1-2 weeks to allow it to heal and encourage it to grow new roots.

Aye, it died. Weather switched around to humid here and it slipped my mind that the mold would maybe be harmful to the cactus in humid weather. The spot where the rot started was on its side right near the the top of soil. The rot infected all of the upper portion quite quickly but didn't seem to infect the subterranean portion farther down with the roots.