I think that's HLLE; although Hippos are known to get themselves in some pretty tight places inside the rockwork. So, it's possible he scraped that side of his face all up.
It doesn't look like the typical HLLE that we usually see, but it's possible. I would add vitamins to his food if you don't already and keep a close eye on it for now. Note any changes in it whether for better or worse.
Looks like HLLE to me, possibly a scrape as well as mentioned. Soak foods in selcon and offer tons of nori. Enough that the tangs can graze for 2/3 the day.