I wouldn't worry about that. I wouldn't even have seen whatever this is if you had not mentioned the adipose fin (that is the name of the small fin just before the caudal or tail fin).
I may have seen similar on my fish, but I'd have to see the fish itself before being certain. If it has been a week and no other spots are visible, it is not likely related to ich. Are any of the fish flashing? Flashing means swimming fast against a leaf, wood, rock or the substrate in an attempt to rub the gill area. Ich always first attacks the gills, and flashing might indicate ich even if no spots can be seen. I still don't think this spot is ich.