need some help with my first saltwater tank

[Sasie0ne]

Hello.. I am just starting to set up my first saltwater tank, I have a 75 gallon FOWLER that is just starting to cycle.. Over the past few days i have noticed i have alot of what looks like .. well cant explain so i will just attach a picture.. can anyone tell me what these sand looking stuff is.. and if they are harmful please..

 

[spoonman]

Maybe sand that is moving with the flow and settling in the shells?

 

[Sasie0ne]

I dunno.. the worm looking things in front of the shells.. i have them all over my tank

 

[Mr. Crabs]

Do they move?