>>1532498do it when you change your chain
you don't need to remove the derailer but unbolt the cage
get a little bowl of kerosene
take all the little bits apart
clean them with a rag/flat head/toothbrush and the solvent
let them totally dry
and re-assemble with a tiny bit of grease wiped around the bushing in the middle
be aware the 2 jockey wheels are different
the top one is supposed to float