no i dont think it damage the plant,i put sulphur on all my lophs that i have cut pups off for grafting and it doesnt harm the plant.red spider mites are one of the uses advertised on the bag of sulphur that i bought from the garden center so it should work well. all my experience dealing with spider mites is on mj plants,and they can be pretty damn resilient. if your soil is really infested(they live in the soil and come out to eat your plants)normal treatments with neem oil,pyrethrin etc wont do the job.but what does knock them back as a last resort is aeresole fly/insect spray with pryrethin added.sounds weird but its safe on plants and really works.