Regarding a standard ultrasonic humidifier, they are generally meant to humidify whole rooms. But since you will be using this in a fridge, which is a very tiny volume, why not consider a mini-humidifier some of which are so simple that issues with mold or harmful bacteria become less of a problem? Let me explain more.
For my curing chamber, a 2.7 cubic foot dorm fridge, I use a donut mini-humidifier. The donut, basically a hollow plastic ring, floats on water, and includes just two other parts--a tiny filter on the bottom, and an ultrasonic oscillating disk that vaporizes the water it floats on. The thing sits in a coffee cup that I fill with spring water, and I top it up as necessary. I keep an eye on it, and it has never become slimy, green, or anything that might indicate mold/bacteria/algae growth. But if it did, since the reservoir is just a coffee cup, well, it would be extraordinarily easy to clean.
If you are not in a rush, you can order these from China for $3, which includes shipping--really! Here is an example link:
https://www.aliexpress.com/item/201...C-5V-Doughnut-Donut-Diffuser/32771582500.html
If you go to Youtube and search, you can find many videos about these things--here's an example:
There is another interesting style, too, that is essentially a cap that you screw onto a bottle of spring water. It has a long filter straw that draws water up to the ultrasonic disk where it is vaporized. The long filter straw can be replaced so this may alleviate potential issues with mold/bacteria/algae. Here's a link to some that goofily look like cowboy hats, not just a screw-on bottle caps. With prices lower than $6, if you can ignore the silly design, it would work wonders for adding humidity to a curing fridge whether screwed onto a water bottle or simply sitting in a cup.
https://www.aliexpress.com/w/wholesale-ultrasonic-humidifier-cowboy-hat.html
There are many other styles available, too--just search for "usb ultrasonic humidifier" on Aliexpress.
Anyway, my point with all of the above is that some of these mini ultrasonic humidifiers are so simple that keeping them clean is much less of an issue than a larger, regular humidifier with reservoir.
By the way, I can vouch that these put out plenty of mist to keep a high humidity level in a mini-fridge and probably even a regular-sized fridge. If folks want more details, a video of one in action, or something else, just let me know.