Is there an adjective for low-toxicity, or can it be used as an adjective itself?
It sounds strange to say, for example, materials that are low-toxicity.
Low-toxic is the adjective of low-toxicity.
If you were asking whether there is a separate word for low-toxic, I am quite sure there is not because there is not a word even for non-toxic. Of course, depending on the context you could say for example "the substance is pure/harmless" when it is non-toxic, but neither pure nor harmless means non-toxic explicitly. So, it is perfectly ok to talk about low-toxic materials when the toxicity of the materials is low.