You probably are aware that tangs have a reputation for being ich magnets. This is a very common disease for tangs.
Hippos like to squeeze in tight places, particularly to sleep. They rarely scratch themselves from this. However, with a new fish, which is probably quite skittish, it is possible that they are scratches. However, if you scratched yourself, you usually would not continue to rub it on rocks. More likely, you have ich.
As long as the fish is eating okay and otherwise seems healthy, you can usually watch the fish. However, in the meantime, you should try to get your new Quarantine tank/Hospital tank partially cycled in case you need to start treatment such as hyposalinity.