I don't have specific advice for cleaning, but the grilles are very easy to remove- they're just held in with a little friction along six round stubs. Pull carefully but firmly and they'll pop right off. I added a picture in case it helps.
Is the fabric discolored, or just not opaque enough to cover the shadows underneath? Your left speaker seems to look about average, but your right speaker is definitely more visible than mine based on your photo. It may be the fabric is overstretched or thinning though.
In bright light those regions are more visible, it might be a bit of dust you could clean with a Wet One/detergent wipe gently. Otherwise, take the grill to a fabric store and see their range of speaker fabrics - you might get a decent match to replace it. The fabric is glued in with contact adhesive.