Rbdx

Hello everyone! Merry Christmas! It's my first time posting on here, and so far - it has been a joyride today! A friend of mine purchased a shubunkin goldfish from my LFS as a Christmas gift, and I noticed that it has an accumulation of black lump near its tail. Here's a video, with regards to the given lump. Is anyone able to identify as to what this is? Much help would be appreciated!