When using Niagara (not sure if this issue is exclusive to Niagara, but it’s where I notice it) I often get these residual artifacts on moving particles when using a masked material. See here:
If I switch the material to translucent, it fixes the issue:
Why does this happen? Is there a solution other than switching to a translucent material?
I think it might be the anti-alias you’re using, try a few other modes. Another possibility is TSR.
There is a node you can use in the material to try and lessen this, called ‘previous frame switch’
You basically feed it what the value of a given parameter would have been in the last frame, to help the system adjust. I didn’t know about it until the other day, or the delta time node for materials… 
Yup, It was TSR causing the issue. Thank you!!
