I try to give an educated guess.
Tethys orbits Saturn outside its inner ring but inside E-ring which is very large.
Here is a photo explaining location of Tethys inside E-ring:https://upload.wikimedia.org/wikipedia/commons/f/f7/Saturn%27s_Rings_PIA03550.jpg
Tethys rotation is synchronized with Saturn so that it faces always same hemisphere towards it while orbiting around Saturn. These red arcs are at the leading hemisphere of Tethys on its North half above equator.
If there is at space a very thin disk of small red objects (almost like dust) and that disk of dust hits a moon (which does not rotate relative to dust disk on same orbit) to its Northern hemisphere it will create an red arc to the surface of that Moon because of gravity. Middle part of dust disk will hit first the surface and both sides a bit later.
Because the dust disk most probably is not totally homogeneous the red arc created to the surface is wider on both sides compared to the middle part which hit the surface first. If dust disk was wide enough (wider than moon) red arc covers the whole moon from one side to another but faints to almost invisible when closing both sides of the moon.
If dust cloud had layers it would have created more than one parallel arc to the surface. Closer to the equator of moon lower arc is closer to the straight line the resulting arc would be.