Damage to the leaf of your cacti could be sunburn if water sat on the leaf when the sun was shining or physical if an animal or falling debris hit the leaf or rarely from an insect that has been sucking sap from the leaf surface....look for insects on the back of the leaf. Is this just one section? Then don't worry about it. If it is all over the entire plant then you could need to move the plant to a different location with less sun or find the hiding insects. You can wipe the leaves with a mild soap and water solution to check for bugs. Just dip a soft rag into the soapy water and wipe. Do not do this when the sun is out. Use an umbrella or move the plant to the shade for a few days after you wipe it. The good news is that although unsightly, your plant should easily survive and out grow the damage. Keep growing, Marianne Binetti