Is this dust?

Does this look like dust or a crack on the rangefinder patch? and how to get rid of it? I read some posts about sending it to CLA but it's just too expensive for that little thing. Anyone has experience with opening the top plate and clean it DIY?

Does this look like dust or a crack on the rangefinder patch? and how to get rid of it? I read some posts about sending it to CLA but it's just too expensive for that little thing. Anyone has experience with opening the top plate and clean it DIY?