you have probably the sample which is moving inside the SEM cavity. you have to know that if the samples are not well fixed, the arriving electron beam can move your samples and then obtained a moving micrographs. note that the displacements are micrometric and perhaps nanometric.

To start with, check the following discussion on RG. It looks like the size does matter.

https://www.researchgate.net/post/What_could_be_the_possible_reasons_for_distorted_scanning_electron_microscope_images_obtained_through_slow_scans

You have charging. Either not enough of coating (recoat your specimen) or bad electrical contact of a specimen with a stub; you may need to apply conductive glue (carbon, silver). 

Dusevich is true, if your sample is not conducting enough, The sem photo will move, you should use silver paste to increase conductivity by applying from the one side to sample holder

As said Dr Dusevich and Dr Altin, it is just a charging. Coat your sample by thin gold layer and check the contact between the coated surface and a stub.

Good luck!  

Thank you all for your answers. 

 It is the typical effect of sample charge: a bright part of the sample together with a dark one. In fact the sample sizes and morphology can affect the optimal coating of the material. Try to redo the coating.

Agree with Serdar Altin and Vladimir Dusevich. You have charge buildup on your sample, use colloidal silver and and draw a streak connecting the Al stub and a corner of your sample, this will ground the charge. Also you can recoat your sample. Since you are using EDAX I guess use carbon. Gold would be a bad idea

This is called beam drift happen when partly conducting and partly non conducting regions across the electron beam thus surface electron is drifted from high dielectric to low dielectric zone and hence the image  seems as moving.

I agree with the other : charging problem here. I don't think it come from the sample really moving, because you use very low magnification. Samples very small movements are observable only at very high magnification and with a slow scan speed.

Several things can cause movement. 1. The sample is not firmly mounted on the specimen stub. 2. Sample is not a good conductor. Charging can cause both real specimen drift and “apparent” specimen drift (the latter meaning specimen charging is impacting the impinging electron probe.) No real drift, but the probe can move a little in response to charge build-up. 3. Insufficient sputter or evaporation coating (carbon metal, etc.).

Cures for: #1. Find a way to firm-up the mounting of your specimen, sometime when carbon adhesive pads are not enough, a little conducting adhesive reinforces (silver paint, or a carbon-based material, etc.)

#2a. A conductive coating is needed, perhaps not just overhead, but down the sides of the specimen as well (i.e. a tilted-rotating turntable within the sputter coater.

#2b. Lower the accelerating voltage, or use a smaller spot size, or use a smaller aperture, or use a low vacuum or environmental type SEM, or some combination of the above.

#2c. Surprisingly, when specimens are small or, ideally, very thin, increasing the accelerating voltage can force a path to ground right through the sample. This has proven quite successful for many low to non-conductors. May not work for highly sensitive specimens.

#3. Using a sputter coater with a rotating, and tilted turntable can help with the process of improving the coating process, especially if the tilt rotates with the rotating stage.

It is charging effect. 

fistly you should decrease the contrast up to lose shining and increse the brightness. if you have still drifting problem, following

if you want to take an image, you have more options. you can coat sample more, or you can decrease the voltage, or you can decrease the vacuum, or you can use options like tv mode, integrate, frame like etc.

if you want to edx amalysis and if your sample contains carbon, you should not coat sample with carbon. Your carbon content can't be correct. you can coat with different material like gold or you can decrease the vacuum.

1. Try @ Low vacuum mode or

2. Check the Alignments: Stigmator & Lens or

3. Increase your conductive coating time or

4. Adjust brightness & Contrast.

Bernardo Frías Bouchez tighten the screw on the side of the stub holder to avoid your image drifting but this normally occurs when imaging at higher magnifications. Also try recoating your sample to prevent charging.