Cracking on slab

Plastic shrinkage cracking. Should have been closed in the finishing process, but weren't. Looks like a mechanical trowel was all the finishing it got. The solution would depend on the use of the slab. If not exposed and not in an aggressive environment, probably nothing needs to be done, just based on the small sample your photo shows.

I agree with hokie66 that you have some plastic shrinkage cracks but you also have another issue that can be interrelated. You have some crazing cracks that are related to the finishing technique. Based on the swirling pattern and the texture of the finish, the surface appears to have been hard troweled with a mechanical steel trowel, also as hokie66 noted.

hokie66 also gives you good advice as to what you can do about....maybe nothing needs to be done, depending on the use of the floor slab. If the slab will have forklift traffic on it or similar abrasive use, you will see a reduction in durability of the surface over time. Depending on the strength of the concrete and the severity of use, that can show up in the first year or can take several years to manifest.

One possibility to alleviate some of that is to use a surface hardener (calcium or sodium silicate based) after the concrete has adequately cured. This material will expand into the pores of the concrete and provide a bit better durability. You have to be a bit careful with its use though, particularly if you have a lot of moisture vapor migration through the slab. You can end up with a "soda ash" mess on the surface if the conditions are not proper.

Kudos for attempting proper curing with wet burlap.