A picture of your plant will help id the problem. If the green leaf had brown icky stuff where it came off the plant and the leaf is a new one up top, this is not good. You may have watered your phal from the top and left water sitting in the top of what we call the "leaf axil". This is where the leaf joins the plant. It usually will kill the plant unless caught soon enough and treated. I use hydrogen peroxide to help with this. If the leaf is a bottom leaf it may not be a big deal. Usually though, leaves turn yellow and wither away on the bottom. If you will include a picture and let us know if it is bottom or top leaves coming off it will help alot. If it is a top leaf...go pour some hydrogen peroxide into the top, let it bubble...pour out and repeat until hydrogen peroxide no longer bubbles alot. Use the tip of a paper towel to wick away left over moisture and place plant where it is shaded and has a nice breeze on it. You can use a fan or a slightly open window...but air movement will be vital.
Good Luck...hope this helps.