Ther reason why GE Silicone II caulk is so difficult to apply, is the very reason it works and lasts so long. It remains pliable and bonds to tile very well.

You do your clean up on your finger tips and caulk-squeegee tool as you go, with paint thinner and a white cotton cloth.

HINT: Use less quantity in the bead before squeegeeing it down the joint. I use a finger instead of the PITA 'tool.'

That entry level grade of DAP is cr&p, losing it's pliability, pulls AWAY from the tile/stone, getting brittle in just a few months.

To answer the collecting of discloring mold in the body of the caulk? That's a tough one. You will have to graduate to something called butyl that is customarily used in exterior and interior flooring and wall concrete and brick expansion joints. You're not gonna find that at a big box or Mom and Pop hardware store. You will have to go to perhaps industrial grade supply house to get that stuff.

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'06 E320 CDI
'17 Corvette Stingray Vert