Fiddler mating?

i have a salt water tank with a male and female fiddle crab in it. recently i noticed theres this weblike nesting on a plastic starfish ornament and on a lil plant in there. is this part of there mating rituals or is this just some sort of algea i need to remove? i havent touched it yet cause i dont know if its thire fry or not but the tank smells and id like to be able to clean it and know im no fry killer. i cant find info on anywhere let alone their mating. please help!!