I take it that the Caulerpa is encroaching on stony coral and for that reason you want to eradicate it? I have had this same problem for going on 2.5 years. The creeping Caulerpas like C. racemosa and C. peltata are among the most difficult plants to eradicate, IME. They persist and continue to grow well when most other problem algae are limited. They also spread readily by fragmentation, and are fragile, making it hard to harvest them in-tank without spreading them. IMO they propogate readily in dealers rock holding tanks, making them frequently encountered arrivals with new live rock, so replacing the rock may not help.
The only grazer that I can recommend for a 18G that might eat Caulerpa is a sea slug. Some sea slugs are specialized to graze siphonous green alga like Caulerpa. They are still rare in the hobby and some slugs are not suitable for high-flow tanks or may get eaten by fish. The only sea slugs I've tried were lettuce sea slugs and these were dissapointing - probably sold past the point where they had stopped grazing, and not able to cope with high/random flow.
Otherwise, its the old routine: remove the rock, wire brush or pressure wash it, rinse, return to tank. Repeat as required. Alternatively, light deprivation for several weeks (rock cooking). If you have coral encrusted on the rocks, obviously neither is very appealing.
There is one other alternative: if you can't beat it, live with it. This may mean giving up on stony coral and prostrate coral like zoos. Its an attractive algae, IMO, and would work well for a seahorse tank or tall soft coral like leathers and Gorgonians.
