I checked now in the morning and it's still the same. It has been 12 hours.
Maybe I should open the rear and make so the telescope points down for some time or should I try clean it with 50/50 Isopropyl and distilled water?
I'd say most definitely NOT!
Because are you just going to soak it in that solution, or do you intend to WIPE it?
First take above advice, leave the thing to completely dry and see if you can blow the thick of it off, currently any solvent is just going to spread that amount to a bigger area if wiping.
You need to find threads or online articles written by people who have regularly cleaned corrector plates and follow that advice, just keep searching.
An immense number of objects can scratch glass which you'd think couldn't. Normally nobody notices and it may not even make a difference to a corrector plate to get micro-scratches, I've no idea. All the same anything that is irreversible needs to be avoided. For example, cotton wool balls are NOT as soft as people think. Even if you can guarantee they are pure cotton. If the stain is stubborn you will potentially accidentally rub harder than you should.
Why you want to water down the iso-propyl alcohol I know not. Is the mix ready made? Is the distilled water really distilled water? I think I heard once a lot of it is filter made and not condenses pure steam, and therefore not distilled water at all. That may be wrong, but check.
iso-propyl alcohol is a good cleaner because it is a good solvent. How much are you going to use? Are corrector plates coated, is there cement glue joining the plate to the tube? I know nothing of these things and they may all be irrelevant, but probably you don't know either.
Again, take advice, let it dry fully. If hairdryer on full blast doesn't shift it see if you can find a can of compressed air that you can test to ensure it doesn't spit out stuff and try that.
Get you finger totally dry and grease clean and try to move it, no nails. Gently, it could have something in it that scratches it, but if you dissolve it and wipe it, even properly, it will spread out.
Stop worrying, you will get it clean. It is natural to worry, and especially annoying when something is new and you think you may have done something silly already.
But approach cleaning very carefully and fully advised, cleaning can cause more damage than the thing causing the need to clean if errors are made, the magic word is "irreversible".
Wait it out and you may finally find a thread or a blog or something where someone has had a very similar problem and successfully resolved, with an explanation of how.
I think the above post that states that it looks like a small drop of dirty water that has dripped onto it and dried is the best guess. The dirt is very black, so could be organic (from the roof), tile dust (from the roof) or often particles from diesel or petrol fumes, of which there is an immense amount in urban areas but most people don't notice it. Think of a van with a dirty back end but none of it is mud, that isn't just splashes and rain. A solvent may not touch these.
This doesn't need to have dropped onto you mirror, it could have been wind-borne.
In short :- let it totally dry and try and shift as much as possibly of it not touching the glass, preferrably using a dry air supply. Find something someone experienced in telescopes recommends for wiping what is left both in terms of liquid and cloth. I don't even know if microfibre is any good! Don't fret, don't rush it. If you have a proper dust cover for the scope use it when done and leave the scope horizontal if not bringing it in till morning. Preferably in the shade to avoid temperature extremes at daybreak should it be sunny.
