Keep in mind that grout and tiles are not waterproof. In fact, both can let water get by them whether they are cracked or not. Grout and tile are there to protect your waterproofing under them and to look pretty.
This is akin to shingles on one's roof: The shingles protect the tar paper waterproofing...and to look pretty.
So assuming you have intact and sound waterproofing for your shower, this is a cosmetic issue.
I want to make sure that you say this cracking is at the joint between the wall and the floor? If so, that is a change of plane and usually change of planes require something more flexible than grout (I assume cementious grout? I ask because some of the new urethane grouts claim some flexibility). Usually, this area should be caulked and not grouted. It may well continue to crack and if you don't like it, you'll have to remove the grout and replace it with a color-matching and sanded/unsanded caulk. The folks at Colorite pert near have every color imaginable to match pert near every grout color out there.