Welcome to the forum, Lenny!
Sanded, cementitious grout.
Looks like a Schluter drain. Which is used as part of a completely waterproof membrane system. Meaning that water isn’t getting anywhere it doesn’t belong unless the integrity of the Kerdi membrane has been compromised.
Ok, grout cracking always means that there is some undesirable movement going on. And we need to figure that part out.
First questions:
1) Are you familiar with any of the construction of the shower?
2) And is the underside of the drain accessible for viewing?