[cmiovino]

For what it's worth, back in ~2012 when I bought my 2004 WRX, the front boot near the downpipe was seeping just like this. I put a worm clamp over top of the existing clamp and cleaned things up. It never leaked after that until the boot itself finally tore about a month ago from age... 12 more years later. During the time, the car was autocrossed extensively doing about 20 events per season for 4-5 years.

When it tore recently, I personally ended up using a Red Hippo split boot kit and repacking it with grease. The car's 20 years old and I feared removing the axle would cause more issues. I'm not sure I'd recommend a split boot kit for track use, even a good $45 one like this one I used, but it worked and has been holding well. Car only sees about 1k miles per year now.

But I digress. Given the boot is fine here and it's just seeping, I'd probably unclamp it from that leaking end and use a proper CV boot clamp (aftermarket or OEM... they should be damn similar). I wouldn't bother with regreasing as you'll likely introduce dirt. You're then also unclamping the other side too.

It's highly unlikely any debris or water is in the boot. I doubt you lost enough grease to cause any issues. The OEM clamps can't be reused. They're also a bit of a bitch to cut off without harming the boot, so just be aware of that. I'd have a boot on hand and be ready to pull the axle in case you damage the boot itself.

Doing the rear axles on these car's (or any Subaru that I'm aware of) doesn't require dropping the diff. They'll slide right out with 2-3 bolts being disconnected on the hub. Check Youtube for the process. This is the proper and best way to do it. The problem is, you're removing the axle, cutting the boot off, literally taking the axle apart at that end to get the new OEM boot back on, regreasing and clamping... and reinstalling. It's a lot and why a lot of people just get new axles at that point.