Being as tile and grout are not waterproof, and assuming (huge assumption) the waterproofing is fine at the pan, the only issue is esthetics. Why you would agree to pay for a new tiled shower enclosure and accept cracked tiles I do not know but that is your choice. Were it me he would be replacing the tiles and the grate screws.

I don't think those tiles are accidentally cracked...I think the installer had to crack them to make the cuts and then put the two pieces back together when they were installed. Other than esthetically, it's not an issue, and I wouldn't worry about it.

As for the screws, they may not be stainless steel, or they may be a poor grade of stainless steel. In any case, remove them and take them to a hardware store where you can get replacements for 25 cents apiece. Bear in mind that you may need either screws (tapered shaft) or bolts (straight shaft), depending on the design of the drain the grate is attached into. And be careful removing them: if you strip those corroded heads you will have another issue to deal with.

Why did the installer use something that would corrode? Because he is a hack who used whatever was in his truck and figured he would be paid and gone before you realized it.

I think the installer probably used the screws that came with the drain cover. Blatantly obvious the the chrome plating has come off of most of the cover with raw metal (brass?) exposed. Was not a quality fitting from the get-go so wouldn't expect the screws to be top grade, either.

Remove one and take it to the hardware store for a match in stainless steel.

ci_lantro-

I couldn't tell for sure whether the drain cover had deteriorated, or if it just looks that way because there are reflections from the OP taking a close-up photo. If you are correct, I second your advice.

Is the drain cover also corroded, or not?

Since the "stains" are not the color of the grout, I'm guessing that the klutzy installer somehow managed to scour off the chrome plating on a brass drain cover when buffing the grout. Since it's brass, it shouldn't deteriorate from water, but you may wish to get an all-stainless steel replacement for the sake of appearance.

if this is a new remodel the screws should not be rusted.. have your contractor replace the tiles that are cracked and replace the rusted screws.

I wouldn't bother with trying to replace those small tiles. They are not causing a problem, and it may be hard to remove, replace, and re-grout them without causing bigger issues (like damaging the waterproofing or getting a perfect grout match). I suspect that you'll quit noticing this minor flaw soon.

The tiles were removed to fixed a leak (which were introduced during shower remodel). i do see water proofing sheet with 4 holes, assuming for prior screws. Contractor said the leak is fixed (i have not tested/showered as i am waiting for the grout cure). is there a risk that the waterproofing sheet was compromised? worried about leaks!

highlighted the 4 holes