From the internet, A cloaca is a common cavity at the end of the digestive tract for the release of both excretory and genital products in vertebrates (except most mammals) and certain invertebrates. Specifically, the cloaca is present in birds, reptiles, amphibians, most fish, and monotremes.
So since rainbow cichlids are fish and are not livebearers, what you see is a cloaca. However, in egg laying fish most of the people I talk with refer to the visible organ as an egg duct or ovipositor and it is very visible on a female about to lay eggs and not visible when she is not about to lay eggs. The males have a similar but much smaller one for sperm and it is probably called something else, but I do not know what. Though there will likely be some suggestions after this is posted.
The internet defines ovipositor as a tubular organ through which a female insect or fish deposits eggs.