I have released the collar. When tightened then slackened a fraction of a turn the micrometer is easier to turn.
Then I think you have found the cause of the problem. You can slacken it right off. If it turns freely then, there is no overhaul needed. I find the micrometer setting does not move on its own in normal use but if you want to be 100% sure you can always clamp the locking collar back down after you set it at the wavelength you chose. No need to lock it with the grub screw though. That is really for use in an industrial machinery where the micrometer might be used as a set stop which must stay in position under vibration etc