It seems used font for µ is missing. Enable below in preferences.
Edit -> Preferences -> Composition -> Substituted Fonts.
InDesign chnaged µg to mg. it was not so nice to have happend
Replace a correct Greek character with a regular 'm'? InDesign does not do that.
Most likely cause: the character in the original document was not a Greek mu to begin with, but typed as a regular 'm' and then a font was applied which made it look like a μ. That does not "make" it a "μ", though - the user was wrong to use such a font.
When you imported the text (you don't say but it sounds like you did) you removed all font overrides, leaving just the proper characters, and so you got the 'm' back that was originally typed in.
I never indiscriminately remove overridden fonts from text without first checking if it's a 'symbol' like font. For example, you would get a similar problem when replacing Wingdings with Times New Roman; what looks like a rightwards arrow in Wingdings is actually the character code for 'à', and so that is what you get when you change the font. If I spot such a font in use, I make sure to replace all 'fake' characters with the proper 'real' ones in that font only (because different fonts may have different encodings) before removing the overrides.