RBDM is one of my least favorite elements, because it's so unnessesarily unrealistic. How hard would it have been to make it release hygn and then fire when touching water, rather than just being an explosive that can also be set off by water?
(I think it was made before HYGN was added, so maybe that's why)
I think your idea looks too complicated, though. (This is mostly because the description is really long)
If you use a shorter description, people are more likely to read it:
MGSM:
-When in contact with WATR, slowly* rusts and turns the WATR into HYGN
-burns slowly when heated (faster with more surface area)
*the same speed that ACID rusts METL
I would also suggest making the rate of hygn production higher than in real life, to make the element more fun to play with.