The term flagstone is used to loosely refer to various types of rocks.
However, with Central American cichlids you would want somewhat high pH (than neutral) and hard(er) water. Thus, the rocks will be safe since if anything, they will maintain or slightly raise the pH and hardness, not lower them. Very few rocks, if any would actually lower the pH. Thus, for Central Americans, the rocks are safe. You should consider the actual parameters of your water to assess whether they are suitable for you fish.