Why is my external SSD constantly heating up on Mac mini with Sequoia 15.3.2?

I've got the same UGREEN enclosure as you, and have macOS Sequoia installed on it as the startup disc. Mine doesn't seem to get warm but I have an old Corsair Force MP510 from 2019 installed in it which is only capable of these speeds...

UGREEN – UGREEN 40Gpbs Hard Drive Enclosure with Cooling Fan

"With the built-in ASM2464PD chip, support for UASP acceleration protocol, and equipped with a USB4-compatible Type-C interface, this 40Gbps M.2 NVME SSD Enclosure is compatible with USB4/3.2/3.1/3.0/2.0 & Thunderbolt 3/4. It achieves lightning-fast data transfers of up to 40Gbps, making it perfect for quickly transferring large files or backing up your computer"

If this enclosure is designed to run using Thunderbolt 3/4 or using USB 40 Gbps mode, you may find that the SSD heats up a bit when in use. That may be why they make a big deal about the aluminum casing (passive heat sink), silicone thermal pad, and active high-speed cooling fan.

If there is no data on the drive, and you're not accessing it, I wouldn't think that it would be heating up, or that the fan would spin at high speed unless was designed to spin the fan all the time regardless of temperature or need.

(Whereas if you had a bunch of data on the drive, and didn't think you were using it, but Spotlight was still building an index, that might be a possible explanation.)

I'm out of guesses.

Most enclosures with a fan have a fixed speed. I have two TB enclosures, one Acasis and one Trebleet both with fans but they stay warm and both have a fixed fan speed. I'm using a WD SN850X and a Samsung 990 pro. Your issue could be drive or enclosure related. You would have to test by swapping out parts. I'm using a Mac Mini M4