There is a rubber O ring that is supposed to keep the grease in. Ok to drive? sure as long as all the bolts are tight and no issue with wheel bearing lock nut backing off. The hub is easy to remove and inspect just take a look and make sure you don't have a bigger problem.
Easy to install? I have to disagree, there are a lot of tools required and a lot of parts have to go out to replace the seals if they're the ones in the back of the spindle.
To the OP: check the link in my signature, you'll find useful info there.
If the grease is coming out the hub cap itself, it probably is the hub cap seal which actually is the very first thing you'll find once you take the hub cap off, replace it with a proper size O-ring and you'll be good to go.
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