When I started up my reef tank I purchased NLS Marine for my SW fish...and I was surprised to find that the ingredients were identical or near-identical to the NLS Cichlid formula I already had on hand. Shouldn't be a problem at all. (after the Marine formula was entirely consumed, my SW fish have been eating NLS Cichlid formula pellets ever since).
I've also fed flake food meant for saltwater fish to my haps and frontosa's when they were juveniles with no ill-effect what so ever. In cases like this, however, especially if you have herbivorous mbuna's, you'll want to check the protein content percentages to make sure they're not too high.