Re your concerns about "cleaning" - it's really important to distinguish what the "definition" of this is.
It is a subject of some confusion and debate among coin collectors - all silver coins will tarnish over time when exposed to the air. I have seen coins with perfect surfaces that were totally black from tarnishing - it is possible to remove the tarnish to reveal the surfaces underneath without doing damage to coin - (and I would suggest that most older coins have been dipped in a tarnish removal solution at some point in their existence).
Is this "cleaning"?
I guess it could be called that - but in a positive way and should not adversely affect the value of coin, unless it is done improperly and has left the surface looking unappealing - or if you would be one of the few collectors that prefer a coin in a dark toned look rather than to see shiny lustrous appearance.
On the flip side, the negative aspect of "cleaning" (which does significantly affect the value of coin) would be "polishing" the surface of coin by rubbing with a cloth or using a powered wheel (whizzing) - this actually changes the surface of the coin by wearing it down with friction to cause it to be more shiny but in fact damages the surface and wears down the details.
I would certainly guess that at sometime in the last 50 years someone has "dipped" the 1900 twenty five cent silver piece you are interested in - it's my opinion this has only enhanced the visual aspect of coin without doing any damage and has exposed very attractive lustre. And in fact makes it easier to accurately grade than if the surface was covered with dark tarnish.