Cuckatoo31 is a special mining algorithm used by Grin known for its high requirements. At first, Cuckatoo31 was not against ASICs and was designed to fight against them in the future. For now, high-end graphics cards by Nvidia and AMD with a large amount of RAM, like GTX 1080 Ti and RTX 2080 Ti, are able to handle the algorithm. The exclusiveness of the algorithm affects mining performance. Grin on Cuckatoo31 was previously the most profitable coin for mining on powerful graphics cards.
At the moment Cuckatoo31 or simply C31 is substituted by the new Cuckatoo32 (C32) algorithm. If your hardware could mine multiple Grin algorithms we highly recommend you check the profitability of each algorithm separately as it is usually different.