How to remove dry grout
Your options at this point are scraping it off or dissolving it.  My first suggestion would be contacting the tile manufacturer.  They would have the best idea of what your tile can handle as far as physical abuse or acid.

Scraping

Scraping is attractive as a primary option because you can go slowly and start in a small inconspicuous area to see if it will damage the tile.  There's no perfect tool for this as far as I know.  I'd start with an abrasive sponge (you might get lucky), steel wool (good for streaks) and then lightly attack clumps with a razor, painter's tool or small sharp chisel.  To limit damage to the tile, you want the tool to be as horizontal as you can make it and still work safely with it.  You should quickly determine how much abuse your tile can handle and if a few scrapes ruins your floor.  Most likely it will be a slow and painful process.

Acid

Sulfumic acid will work if the grout is not too thick, the main concern is what it will do to the tile underneath.  Again, find a small spot to test first.  Etching is usually caused because the tile absorbs the acid.  To limit this, I would wet the tile pretty thoroughly with clean water first.  Gloves, mask, and good ventilation are a must when working with the acid.  If the grout is pretty thick, this may loosen it up enough that you can scrub it away with a scraping method.  Follow the instructions and don't let it sit long enough that it eats through the top layer of tile.

Replacement

I wouldn't overlook replacement as a possibility, depending on how many tiles are an issue.  It's entirely possible you will damage the tile trying to remove the grout anyway.  I'm assuming you did the tile work (or you should be calling the contractor to fix this), so you should be able to replace a tile, just break it, scrape up the surrounding grout and install a new one.  If it's old work (and you can find tile that matches), it might be worth a few calls to find what a tile mechanic might charge for a small job.