Hello & welcome. I do apologize that my colleagues could not aid you while I was away. Still, I am Dr. B, a licensed veterinarian and I would like to help you today.
If you would still like assistance, can you tell me:
How long has the spot been there?
Does it look fluffy or like a wound?
When did you last test your water? What are your pH, ammonia, nitrite, and nitrate readings?
Can you take a photo of this? If you can do so, this will let me see what you are seeing. To post them online, you can use the paperclip icon on the tool bar above your text box. Or if you can't see that on your computer/phone, then you can post them on any other site (ie Flickr, Photobucket, imgur, etc) and paste the web link here for me to check.