The grass is an elemental that they are. The grass. they are friends with Rocks. but the same thing. Lava is dangerous for grass either!
Only Rocks gets infected after touching or being attacked by lava that makes their skin lava and makes them agressive. the cure is water for infected rocks.
it happens like that to grass either. but they are not being infected by Lava. the are dying when touching The lava or the infected Rock By Lava if the Grass touches.
They are also scared of Lava Like Rocks