The anti-mildew additives in caulk leach out over time. The presence of mold around your tub indicates that the anti-mildew stuff has pooped out. The mold and cracks are telling you that it is time to remove the old caulk and replace it.

Sashco makes a caulk called 'MildewFree' with the claim that the mildewicides will last for seven years.

It's not easy to remove silicone caulk. The method I've had success with is to use a single edge razor blade to cut it out (you'll need multiple blades as they dull quickly). Then use a scrubbing pad and some Ajax cleanser to go after the remaining residue. Once it's cleaned up and dry, you can re-caulk.

I just caulked a new tub area using Lexel. It's not as pretty as silicone, but lexel sticks to itself. So if for some reason the tub ever needs resealed, it will be a much easier job. I hope to never ever have to remove silicone in my life ever again.

I removed all the caulk about five years ago and recaulked it with Polyseamseal. It is not silicon. I was surprised how well it turned outl Last night I ended up using this over the cracks. It actually looks perfect. I'll see how long it lasts. I'd really rather not redo the whole thing.

Then I realized that the mold was exactly where these cracks were. Now I'm wondering if there is a big mold problem behind the wall.

The mildew is always where the cracks are. That's where the moisture stays the longest.

polyseamseal is okay. I've found it get's really brittle within a couple years and cracks in a shower/tub environment.