I had an "Unknown" app using my microphone on the Apple Watch from time to time. Turns out it was the handwashing detection. Nowadays it's labelled correctly.
This can also be an app that you (very) recently deleted, but as the dot is still showing it's indicating that the microphone is in use right at this moment. I would assume it's a system service that isn't properly labelled (maybe something to do with iOS 18 being so new and it was an oversight), but I don't know which system service would need to use the microphone.