Welcome to the site!
Your temp is good. Anything above a 2 degree swing may cause problems.
You will want to add a ph for surface ripple. In SW
exchange happens at the surface of the water, so good surface agitation is important.
How old is your test kit? Those readings seem a bit off, unless you took them after you added the crab food.
A cycle should go like this:
1. Spike in ammonia
2. Spike in nitrItes with the decline in ammonia
3. Spike in nitrAtes with a decline in nitrItes
4. Steady reeding of nitrAtes with ammonia and nitrItes at 0
The brown you are seeing looks to be a diatom bloom (perfectly normal) and will burn them selves out.
Since you have rock from the beach, I would get a copper test just to be safe. You should also seed your tank with some LR
so the coraline will spread.
You may want to check your source water for ammonia and nitrAtes.
Once you cycle is finished, you can do a few 10-20% PWCs ro
reduce nitrAtes or a 50% PWC
to see where your nitrAtes are.