Hello, We have a 2 piece shower unit with a gap between the wall and the tub. Should I use silicone to fill this gap? The gap is 1/16 to 3/16 of an inch and water goes in and lays in the back of it. We have trouble cleaning this gap. I have not used silicone because I am afraid mold or mildew will get in this or in the silicone.
Thanks.

I'm guessing this is a fiberglass tub and wall unit? You really don't have to caulk that at all if it is a fiberglass unit. There is usually a nailing flange on the top edge of the tub unit about an inch high. Then the wall unit rest down on top of the tub unit flange to prevent water from getting behind the tub unit piece.

You can use a mildew resistant latex/silcone based caulking for tub/tile application that will work if your worried about the gap. Clean and dry out the gap and then apply the caulking.