You operator is right, its debris. Specimen issue. You need to polish your specimens again with good intermediate and final washes. Also you may want to use higher accelerating voltage (like 15 kV instead of 5 kV) for better contrast.

As far as phase analysis and evaluating binder distribution goes-- I would recommend that you use different instruments/techniques that can provide you much more detailed information of what you are looking for. I recommend using EDS/EDX to get an elemental distribution profile and/or some sort of XRD to get phase characterization profile. I used both of those x-ray techniques in addition to SEM when I was analyzing the elemental distribution, phase composition, and grain morphology of my materials and I was able to get clear and comprehensive data about my material. You might be able to use SAXS/WAXS but I have not yet used these techniques enough to know the full advantages and limitations of them. I have only used SAXS/WAXS a few times but these techniques gave me very in-depth data that helped me at the time I employed them. Still, they might be worth considering. When/if you get rid of the debris, or specimen, issue and improve the quality of your SEM images then I would say that you can use some sort of image analysis tool such as ImageJ to analyze your grain boundaries more closely. You might not be able to get good EDS/EDX data until you sort out the debris/specimen issue but you should be able to get good XRD data for your phase characterization since it does not rely on "the picture" that the electrons paint in the same way that the SEM and EDS/EDX do. If you troubleshoot the issue and are still having trouble getting clear SEM images then I would say to still try and get EDS/EDX just to see what it might tell you. Alternatively, since your material appears to be in clumps rather than a monolayer of easily visualized particles, you could try a different imaging technique such as TEM, which might allow you to distinguish grain boundaries more easily. Hope that helps!

Vladimir is wright. I suspect the surface contamination may be polishing media. I often find that a simple wash step is not enough. It helps to wet your thumb and do a gentle wipe with your wet thumb while under the stream of running water. I then generally check quality of polish and cleanliness of surface under a reflective light microscope. Such surface particles are generally visible with that light microscope If they are present. It saves waisting time on the SEM.