You can't cover it up. You need to scrape it all off, ALL OF IT. Thoroughly clean the tub, and wall surface with something like rubbing alcohol, or bleach, then buy a tube of caulk. They sell some brands that look like a big tube of tooth paste, or the kind that goes in a caulking gun. Make sure it's for bath and kitchen use, and it's water/mildew proof. It'll cost you about $6 bucks per tube, you may need two. Cut the tip of the tube, about 1/4" down from the tip, at a 45 degree angel, and hold the tube, at that same angle, so the slashed tip is parallel to the surface you are applying it to. Start at a corner, and applying an even steady pressure, move at a steady pace, from one end to the next. When you get to a stopping point...a corner, or something, wet the tip of your finger and smooth the bead of caulk, wiping the extra off on a rag, you are willing to throw out. Clean up any caulk that gets smeared on the tile, or wall before it "skins" over. Do this over again, until all of the wall/tub seams are caulked. Then, let it cure for at least 72 hours before using the tub.
I just did it...