Just as Drewco said, it doesn't take long for it to take off. We had very little on our rubble in April last year. The lava has only been in there for about 8 months. Now ALL of the lava rock and all other rubble has it all over, but only on the top portion that is exposed to light.
I did not know, however, that crabs LOVE it. They have totally eaten it off one entire large rockface. There are a lot of people who do not like to add chemical stuff. If you have patience, it will be all over in a few months naturally, without any possible harm to your trigger or anything else in there. Good luck, and be patient, it will start to appear one day and just take off.