I think this shot has fallen into the same trap that one of mine did a few months ago - I posted about that one here
I would agree that I cannot see any dirt in your photo, but that patch in the cloud highlighted by colleagues above does look rather like dust spots/blemishes look in digital images. You can see why someone might be fooled into thinking it was dirt.
I raised the issue in my thread about what we are supposed to do in such circumstances. There was no definitive answer, so we are left to draw conclusions but, on that basis, it seems that the least we should do is mention the issue to the screeners, but in the end it appears that such 'apparent' dirt should be cloned out. This may appear to contravene the sites rules about cloning, but it looks as though having a clean looking image takes priority. I drew this conclusion because I appealed my image - after being able to demonstrate beyond doubt that my 'blemish' was in fact a wisp of cloud - and it was still rejected for 'dirty'. I received no further message but on that basis I can only conclude that the screeners wanted me to tidy up the apparent blemish by cloning.
It may be helpful to get confirmation of this from the team - or a different view if required.
All the best.