Move a potted plant that has been resting directly on an unprotected concrete surface, and you will likely find an unsightly ring. Iron from the soil and from the fertilizer in the soil leaches out and oxidizes upon contact with moisture and air. So, the ugly marks underneath potted plants are likely rust marks. Refer to the article in this section for specific directions on how to remove rust marks from concrete using a solution of Bar Keeper's Friend and water. It is still best to test any cleaning solution and technique in a small, inconspicuous area first to verify that it's effective and won't have any undesirable results on your surface.
To prevent subsequent stains, use a protective pad or dish between any plant pots and your concrete surface.
Caution: Never mix cleaning agents or chemicals, the result can be dangerous or deadly. Before cleaning, always test the agent on an inconspicuous location to determine its suitability and to make certain it does not damage the material. Wear appropriate clothing such as gloves and protective eyewear, and work in a well-ventilated area. Accidental inhalation or ingestion of cleaning agents can be hazardous and even fatal, particularly to pets and children.