"Scuds" are actually an amphipod
....generally they are detritus consumers. One of the good guys!!!
My bet is that the moss is already dying (in part, or in whole) and they are just cashing in on the rotting tissue. Doing you a favor in the process!
FWIW, in saltwater tanks (they are also found on land - circumglobal!) they are a treasured addition for their ability to get into places to clean that nothing else is able to....and along with some other saltwater detritavores are also blamed incorrectly for killing something that is already dead/dying.
There is the (somewhat unlikely) possibility of growing a large population of amphipods in a relatively untended tank, then making changes to the system that eliminate a large share of their food (i.e. cleaning) which could cause starvation. The larger amphipods in saltwater when in a starvation scenario have been known to eat the smaller "ephemera" from live, non-dying, corals. Could be the case with your amphipods and moss. This should be a transitional thing as the population naturally declines, but yes they will try to survive however they can in the mean time.
Personally I would do my level best to keep them around and keep them happy due to their role as excellent cleaners......consistent (regardless of level) cleaning practices on your part should keep the population in control and keep them from "getting aggressive" again. Again, consistency, more than completeness or other way of analyzing your cleaning routine, will be what makes the difference if you wanna go this route.
Also, while I haven't experimented, I'd be shocked if there weren't an appropriately sized fish (barbs, tetras, danios, loaches...maybe some cichlids?) that could assist in control or even eradication. Of course consideration for your other inverts is a must in this scenario in terms of getting a fish which stays small enough. Wrasses are what people in saltwater typically use...and while officially unrelated to cichlids, their look, behavior and feeding behaviors are often quite similar, FWIW.
Wait...your amphipods don't look like this
do they????? You're screwed!