What the...? A Mushroom Mystery

We have a quasi-terrarium that we keep 3 types succulents in. We've had them since the week before Christmas and I've only managed to kill one; that might be a record for me. Anyway, the other day this weird thing sprouted from the soil next to one of the plants. Figured one of my plants was reproducing! That was exciting, except it didn't look like the "succulent sprout" pictures on Google Images. It looked more like a mushroom. And then the next day, it grew some more into what was definitely a mushroom.

Now, correct me if I'm wrong, but mushrooms generally grow in wet, shady areas, right? We keep the terrarium soil pretty dry and the whole thing in pretty decent sunlight since, you know, they're succulents. Kind of desert-y and whatnot.

(Taken this morning)

What the heck is going on here?

(This was taken last night under a lamp)

I blame the gnomes that do our dishes.

Any mushroom hobbyists? Experts? I spent my time in AP biology writing notes to my friends so mushrooms aren't really my thing.