thats the iron sulfate in it, (ferric, anhydrous)... Walmart has cheap flow master 1 and 2 gallon sprayers on sale right now for 5 and 10 dollars, they work fine, you can get a backpack one for around 60....

id just get a small 1 gallon for the moss out, and a nicer one for the weed killer...

as you know Round Up and WBG (24D) mix just fine, and kills pretty much anything in its path... if you can get Crossbow or something similar you will have better results than with just the WBG but you probably already know that... its a tad more expensive as well, but that and Round Up kills everything....