looks like some kind of rot or fungus. at the base, is there some there? (lighter yellow boundary at the very base)
i'd suggest lightly scraping it wherever the lighter color blotches occur. they should come off as a light dusting, although you may still have some discoloration left. you can use a very soft bristle (new/clean) toothbrush or even a blunt pencil (i've used chopsticks, tank-only) to rub/scrape similar occurrences. you're not trying to removing the top layer of flesh, more like just rubbing off a dried scab.
i'd add a little flow to the area or slightly move it into an area of greater flow as it will likely slime up a little in response and the increased flow should help the situation anyways. i also usually treat with lugol's (2~3 drops for a tank that size per week) but that's pers pref.
if you can siphon out the scrapings, it'd be better but i realize not always possible.