Most likely mould of some sort, you should be able to wash it off with soapy water (use dish washing detergent) and then rinse it with distilled water.
I wouldn't say mould. Mould wouldn't necessarily be along the edge.
I would say it is corrosion of the aluminium coating under the silica overcoat. It is an unfortunate issue with these mirrors sometimes. Very much batch dependant as to which mirrors fail this way. I've seen 10 year old GSO mirrors that are near perfect, and others that are shot.
If it is mould, and it is only on the surface, a very careful clean like Bill suggests will remove it. There's an article in the "Projects and Articles" heading in the left margin.
If it is corrosion, cleaning won't remove it. Corrosion will also have a slight scalloped shape to the corrosion pattern. Corrosion can only be fixed by stripping the coatings on the mirror and recoating the substrate glass, which is not a cheap process unfortunately. The only other thing to do is acquire a new replacement GSO mirror which could be less expensive than recoating this mirror.
As Alex has mentioned, this is quite a common occurrence of older mirrors, the Aluminium coating is starting to oxidise and will need replacing to bring it back to new.
in the case that i cant wash it off and have to get a new mirror...
1.) are there better quality ones that will fit the scope?
i'd rather get a better one if a GSO direct replacement is going
to fail again after 4 years
(its a 200mm f5)
2.) understanding that a re-coat is more expensive,
will i end up with a better quality mirror than can be bought new?
3.) Do i really need to worry at all? is it likely to get worse?
worse? probably, humidity is likely to have encouraged it to start and so will continue.
I don't know if there are any aluminium oxide inhibiting paints (try a car or marine store) you could take the mirror out and carefully paint an inhibitor and/or latex paint layer around the side edge to slow it. if it thickens around the edge it might seperate the layer and ruin the mirror completely.
if the mirror has a metal ring around it you can probably remove that or attach a sacrificial anode to it (one of those cheap light metal pencil sharpeners as they are made of magnesium attached to the ring should corrode instead of the ring). I don't know whats in your scope so hard to know, it may have started with metal clips attached to the mirror too. dismantle and see if step one no matter what.
as it stands its not a significant amount or loss of quality to be noticable .
