Silicone will start drying before you finish applying, tape will pull it off more the longer you wait. I don’t use tape My choice 1 -That needs to be cleared out first. 2-You can use a silicone grout that matches the existing grout or Try Amp caulking very little sag. Just a simple reminder there’s a 1/4-1/2 “ lip extending upward on that tub you can’t see. Curious if that sub floor is sagging