I agree with those above, you need to clean your shower. This is my recommendation: Scrub Free is great on soap scum (not to be confused with soft scrub). Spray, wait 3–5 minutes and wipe clean. Repeat if you have to. Then rinse. Tylex is good on mold and other germy things, just spray, wait several minutes, then rinse. Do not use the two chemicals together, you need to do the two different things a day or two apart. While waiting for the chemicals to work after spraying leave the room, you can wear a mask if you are sensitive or worried about the chemicals. Once you get it all cleaned up you can use more preventative methods, and stick to more natural remedies if you prefer not to use harsh chemicals.
If the mold is very stubborn, there is a chance it is living behind the tile. If a shower is constructed poorly with inadequate measures against mold, the dry wall behind the tile might stay damp in between usage and harbor mold. But, usually the mold is in the grout when that happens, not on the tiles in soap scum.