a) don't get hit by those caustic missiles
b) don't fly through the caustic cloud when a thargoid blows up
c) If you do get some of the caustic stuff on your ship, you can remove it by:
1) heating up your ship (silent running is one way to do this, or pop some SCBs, etc -- anything to get your heat up for a while, until the caustic burns off)
2) using decontamination limpets, which will remove the caustic and repair your hull
3) high tailing it back to a station -- the caustic will be removed when you dock.
That's all I know.