Technically, you are NOT allowed to bring it back into the US because it does have micro organisms. Most of the time they will allow it, but if you bring more than a handful or two, they can dump it before you get on the flight.

Legally and ethically, you should avoid taking sand from the Bahamas. While many travelers are tempted to scoop up a small vial of the famous pink or white sand as a souvenir, doing so can result in it being confiscated by Customs and Border Protection (CBP) when you return home. Most countries, including the United States, have strict agricultural and environmental regulations (such as those enforced by the USDA) that prohibit the importation of "soil or organic materials" because they can harbor microscopic pests, bacteria, or invasive species that could harm domestic ecosystems. Furthermore, many Bahamian islands have local regulations protecting their natural resources; removing sand contributes to coastal erosion, which is a significant environmental threat to low-lying island nations. If you are caught with sand in your luggage during a search, you could face delays or even fines. A much better and legal alternative is to purchase locally-made handicrafts, such as straw-woven baskets or certified "Bahamian sand" jewelry, which has been sterilized and processed specifically for export and complies with international travel laws.