Thanks guys!
Ed, I just checked the tank and could see no such invaders elsewhere in the aquarium. No, they're not always mobile. Guess this might mean they're parasitic. If they're only feeding on dead skin particles and perform a cleaning functon, then they're welcome guests. Otherwise they'll be "terminated with extreme prejudice"!
Ralf, no, don't intend to go on a turkey shoot to rid myself of them for fear of "collateral damage"
I never knew what a "turkey baster" was either until recently. In Japanese, it's called a "spoit" (an imported word, I think). I used to call them "spoits" when speaking English until somebody told me they hadn't a clue as to what I was talking about!