They need something much heavyer: Mysis shrimp, or chopped squid or clam or fish. Make sure each polyp gets a good mouthfull every other day (or more if you want) and they will grow like crazy.
Placeing them in a cave is a sure way to a failure because it is near impossible to get enough food back there. Although they don't need light, they are best, out in the open, near the surface where it is easy to feed them. Because the easier it is to feed them, the more likely you will do it.