Dots on iPad screen/glass.

Aside from surface dust, the marks to which it is assumed that you refer may be residue from the previously fitted Screen Protector. Some poor quality screen protectors can leave adhesive residues on the glass surface.

Another potential issue is damage to the iPad's screen coatings - which may be caused by inappropriate cleaning methods. Apple's recommended cleaning instructions can be found here:

How to clean your Apple products - Apple Support

If the marking that you see is objectionable, you might consider having your iPad screen assessed at the Genius Bar of an Apple Store or Apple Authorised Service Provider (AASP) - noting that a fitted screen protector will be removed for the assessment. Screen protectors, when removed, should be discarded - as the adhesive will often be contaminated with dust and other detritus and will rarely properly re-adhere to the screen.