This is serious. If you do nothing it is going to get worse.
I would not sand or grind to investigate, I would do the following:
Unbolt keel, chock it, and raise hull clear.
Flop keel on its side.
Lower and chock hull.
Replace all keel bolts, if you must use stainless use 304.
Refinish the mating surface on both keel and stub.
Use grinder, and Ospho to kill rust on keel
Paint top surface with Epoxy
Raise hull and position keel for mating
Mask from just below the joint for 12" with paper.
Use 5200 and cut the cartridge off so that the whole turd comes out. (Prob 5x carts)
Spread the sealant with spatula down the centerline but more than 1" thick
Smear sealant up the bolts filling the threads no bubbles at least the thickness of the hull stub.
Drop boat onto keel, this will squeeze excess out and up through the hull holes =good
Scrape excess ooze on outside and inside on bolts.
Put new washers and load spreaders and one nut and tighten to just past hand tight
Once cured, torque bolts to the rec. torque for the diameter and matl of the bolt. (search online)
Sand excess sealant outside and then epoxy prime a continuous coat from the keel up onto the stub 3x.
Epoxy barrier coat eg Hempel Hi-Protect - antifoul
Place second lock nuts on bolts.
If Stainless bolts, clean the exposed metal spotlessly clean.
Drill holes in keel tap threads and fit anodes with bolts
Paint the backside of the anode in epoxy and anti-fouling
This will not go away by itself.