I have removed biofilm from solid surfaces by immersing in a known volume of sterile saline, sonicating in an ultrasonic cleaning bath, serial diluting then spread plating. Haven’t done it with psychrophiles but most stages could be cooled. Sonication period would be short.

I do not see what would happen with the Psychrophile. I didn't know their colony forming units needed a different way to be calculated. Never heard of it. It is just a temperature thing. Incubate your plates at psychrophilic temperatures, the ranges which will be determined by you. Colonies that develop at those temperatures are psychrophilic colonies and a combination with what Jim C Philp has said should solve your problem

Also ~ watch out for condensation on agar plates incubated at low temperature. Dry plates well first and incubate upside down.

I think, psychrophiles need 5 days in plate count.