This is why tile sucks in a shower. Dig that crap out and fill it with 100% color matched silicone.

I am creating a second story bathroom with a walk in shower with (hopefully ) tiles . Just like everything you do there are multiple answers . I have done a lot of research and some say grout is fine if you have a sturdy framed shower but others say you must use silicone due to chance of grout cracking . The down fall of silicone is color and it must be redone after 5 or so years of use . That being said most likely looks like grout so you'll have to scrape it out and then I would replace it with a close silicone color . Look it up on youtube , they have some imformative videos.

Joseph - Not necessarily so. All depends on the grout.

I'd scrape out all that nasty cementitious grout from everywhere in that shower and re-grout using non-porous epoxy grout. Worth every penny. Epoxy grout never needs cleaning.

We've used Spectralock Pro 10 years ago in our master bath and were so pleased with it, we used it 7 years ago in the kid's bath too. Both bathrooms get used everyday. No discoloration, no mold, nothing. The bathrooms look like we put them in yesterday. Love the stuff.

Good luck!

Grout on flat surfaces. Caulk in the corners where there is always more movement possible.

From the TCNA site:

When do you use caulk instead of grout?

Technically, anywhere there is a change in substrate or backing surface such as the joint between walks and floor and wall joint, caulk should be used in place of grout since these surfaces move independently of each other. However, it is important to recognize and make the end user aware of some important points.

Often, installers use grout in place of caulk for these reasons:

1. The caulk may not exactly match the grout color.
2. Even when the caulk exactly matches the grout color when installed, it may not match six months later (caulk will "age" differently from the grout).
3. Caulk will need to be maintained more often than grout.
4. Mold may grow more easily on caulk (except caulk treated with mildewcide) than on grout.
5. Acrylic caulks break down in horizontal wet applications. Silicone, urethane, or multi-polymer caulks are better choices but can be harder to apply.

However, when grout is used in place of caulk, the grout can cause structural and aesthetic problems.The grout will crack allowing moisture to penetrate.Where the grout is sufficiently strong, movement in the walls, floor, or countertop can damage the tile.Grout cannot hide corner cuts as well as caulk.

In summary, caulk is the better choice, but the customer needs to understand its limitations.

https://www.tcnatile.com/faqs/45-sanded-grout.html