My sea moss should last a year. Every person has different needs for dosing, but a few spoons a day should be enough. Cara and I take a large handful of our smoothies every day. Sea Moss grows in the North Atlantic and in the very southern part of the South Pacific where the sea is very clean. And yes, it has been tested for heavy metals. You shouldn't worry about this, since all seaweed has algin.
Algin binds heavy metals and removes them from your body. You will never be contaminated with heavy metals from marine algae, even if it grew in radioactive waste (which my Sea Moss is not anyway). Yes, it's just sun-dried and packaged. There is still ocean debris tangled in it and it smells really strong when you first open it. Therefore it is necessary to wash it really well several times. It expands up to 4 times its size, with fluid.
Even if you use it daily, it should last a month, unless you use it in a lot of recipes like Cara and me. It is normal for the Sea Moss to show colors from white to black and you can find white powder on it.
These are marine minerals and are nothing to worry about. In case you're wondering, Sea Moss is the type of collagen that builds up the ligaments and cartilage between your joints.