wait! the gap between the tub and upper surround should not be caulked generally.
if it's tile, the tile is not a waterproof barrier, the waterproof membraine should be behind the tile and this gap allows water that gets behind it to drain out and air to get in. I have recaulked and repaired grout and tile showers that when caulked improperly and the then removed, gallons of stinky water drain out from behind the tile.
when fiberglass or similar this joint is normally "shingled" or lapped so that water would have to travel several inches up to get past this joint. when its left open it provides a gap that can dry out instead of leaky caulk that traps water behind it and molds.
give us some pictures and more details. your situation could be different but its likely this should not be caulked in the first place.
and
@Greenman is correct about silicone. very few correct uses for this product. I'm a fan of OSI quad and their other products. SashCo also makes some great sealants. just remember, caulk is not a waterproofing strategy, it's a band-aid.