How do I denoise a microscope image?
I am also not an expert, but I would consider this: The 50 µm watermark is burned in the image and it is complete black (0 0 0) in rgb. It has a significant influence to any analysis oyu make on the image regarding statistics like mean, median, edge detection etc.

So it needs to be removed. The easiest method to remove it would be to find out if you microscope has a setting to metadata this information instead of burning it in the image. 
If that is not available and you only have this image provided, then I personally would go as follows:


Segment the black "Noise" watermark area and 
use that selection as a mask to fill with the mean color.
Do your analysis. Be critical of whether it influences the results and refine if necessary. (E.g. if it still interferes with edge detection add some random noise).


Either that, or, if you can afford it - exclude the area of the watermark altogether from the analysis.

But please correct me if anyone knows better.