Crocodiles will attack a hippo calf if it’s too far from an adult, but not an adult hippo. And the reason for that is simple; an adult hippo will kill the crocodile.
Nile crocodiles typically weigh a ton at the most, while a hippo can range from a ton and a half to two tons. As a general rule a carnivore doesn’t want to pick something its own size, let alone something bigger than it is.
A crocodile has a stronger bite but the hippo can easily open its mouth wide enough enough to encircle the crocodile, impale it with its tusks and crush it. Meanwhile a hippo is so big its hard for anything short of an elephant to do enough damage to kill one.
A fully grown hippo is just plan too big and too powerful for a single crocodile to even think of attacking. Hence why they only feed on an adult that’s already dead, but hippo herds can sometimes get attached to dead family members and try to defend the body. Only a very large group of crocodiles attracted by the meat will drive the herd away. That’s the only time you’ll likely see the crocodiles winning against hippos, other times even if there are numerous crocodiles in the same place as a hippo, they will still leave it alone since they are solitary animals and none of them wants to give their live so the rest can be fed.