No problem. To cross just the border by land you do not need anything but your green card to come back to the USA. If you want to go further down check with mx immigration when you cross into Mexico. You may need a permit to continue further south. If you are flying in then definitely you need a passport of your country in addition to your green card. Visa exemption depends of the treaty that your country has with Mexico. Check with your USA Department of State equivalent in your country for exemptions.