Is your plant getting sufficient light? Prayer plants should not be placed in blazing sunshine, but they like a lot of indirect light. Too dark conditions will make new leaves lose color,

Liable to be a flowering crabapple.

Are tile installers not supposed to discard tiles that are damaged or otherwise don't fit in with the overall look of the project? Not every tile in a box can and should be used. If the tile looks odd and out of place next to the majority of the others, throw the darn thing out or use it for a cut. Common sense. I can only see using that tile if the installer was down to the last tile and there were no others available. It looks bad because it doesn't look like any of the others! And it looks bad because it looks like a cracked tile, whether it is or not. To go ahead and use it with the excuse that is a "natural" material and therefore normal to be flawed seems kinda thoughtless and lazy. I'll bet if the contractor uses photos of that job on his website, he will edit out the red vein. He's not likely to highlight it as a "natural" feature. Unfortunately, it is highlighted in this install ... lined up perfectly with the corner of the window. A lovely bathroom, and your eye goes straight to that crack.

Cercospora leaf spot. Info here. Sorrel is in the same family as beets and chard and can get the same diseases.