If it is a water mark it can be removed by carefully warming the area. Techniques include using a hair drier to gently warm the area making sure not to overheat it. Another involves placing a DRY lint free cloth pure cotton cloth (no nylon which might melt), over the mark and using a clothes iron on a low DRY (no steam), setting and warm the wood by gently ironing it through the cloth. An internet search for ‘how to remove water marks from furniture’ will tell you how in more detail. Always start with the most gentle technique and work upwards if it doesn’t work.