I'm not an tile expert by any means, but imo that is definitely going to need to be decoupled.

It's a not a huge sub, but r/tile does exist, try asking over there.

You should fill that crack, preferably with a polymer-based filler. Then as this is a bathroom, you should have sealed the whole floor with a tanking kit before tiling. That way, if water finds its way through cracks in the grout, it won’t find its way through the cracks in the concrete.

I would make it the expansion joint in my tile.

Expanse of tile greater than 15-20 feet should have an expansion joint anyway.

I think you’re right that it’s a crack.

I’ve never seen such a perfect crack in a foundation before.

Stuff like redguard is rated as a crack isolation layer.

You could also use a purpose made crack isolation layer like crack buster that’s rated for mosaic tiles.

https://www.custombuildingproducts.com/TDS/TDS-103.pdf

Looks more like a settlement crack. I would expect to see a perfectly straight line and expansion joint material in the joint. Too many irregularities in the crack to be made on purpose.

You may find the tile will also crack over time if the slab continues to shift.

It has a white plastic strip embedded in it. The irregularities come from Thinset that was used to fill the crack a long time ago it seems.

Saw someone else mention some kind of membrane. Not familiar with that although I have seen Mike Holmes use an orange material that gets thinset to the floor then the tile laid on that.

Schluter ditra. Was going to use that but the tile needs to be at least 2x2 which this one isn’t. Might have to change the tile I use

Even if your tile comes on the mesh backing? Doesn’t look like individual tiles.

You need and isolation membrane. There’s things like ditra that go over the entire floor, or there is fabric sold on rolls that is around 6” wide and applied to only those areas.

You really need to use a waterproofing system before the tiles.

You need to honor your expansion joints. https://www.ceramictilefoundation.org/blog/honoring-expansion-joints-in-concrete-floor

No uncoupling membrane under that tile?? Yikes

Edit: you said Ditra isn’t recommended for small tile but there’s other options. Green skin, other isolation membranes, add subfloor with DMX and tng osb before tile and tapcon to the floor, etc

That’s the way I’ll go, redguard has a system without the size limitation

Both ditra and kerdi board require at least a 2x2 tile to get the warranty from schluter. Both substrates aren’t stiff enough to resist the point load that could result from weight on small tiles.

When you get bigger tiles, the tiles themselves, and the thicker thinset bed, make for a stiffer assembly.

Is that tile recommended for floors? Looks like a wall tile to me and even if it is for floors that’s a lot of grout area to keep clean which is the number one thing most people dislike about ceramic tile….keeping the grout clean

It’s marble, a bitch to keep clean by itself :-) works fine for flooring though

If Schluter won't work, try Widi or equivalent.

I used red guards decoupling mat under my shower and bathroom tile. It also say "recommended" minimum of 2" but I've had zero issues in 6 years with 1x1 mosaic tile in the shower. Personally I'd take the risk on the smaller tile which "might" be a problem later with cracking grout over not using a decoupler that "will" be a problem over that concrete joint.

The only other option really is to put an expansion joint in the tile. But that is going to be right in front of the toilet where you will feel it all the time with your feet. I'd throw down a membrane and worry about possible tiles moving down the road. Unless you are wearing high heels in that bathroom a lot with a point load on a single tile it's not ever going to be a problem. You'll lose any "warranty" by using small tile but honestly you are putting tile over a concrete slab... Water getting thru isn't really a problem.

That’s what I ended up doing. Was considering doing an expansion joint using silicone but decided to just risk it and deal with cracked grout later if it becomes a problem