It's in my DT, originally visible as of now only partially.
For SW tanks it's amazing how little of an amount of living vreatures have attached to this as opposed to the rocks.
Good point on where you collect it, I got mine off Strech Island in the sound, less contaminates, and remember this was 30 years ago
Once you have boiled it or pour boiling water over it, put it in a 5 gallon bucket for a few days (with water) to see if anything starts leaching out.
You could also take PH, amm, nitrite and nitrate readings of the water before putting it in the bucket and a couple of days later to be safe