the red circles are around visible portions of the hawaii base rock in this photo, it is more visible form the sides, but as i wanted to showcase my 7 bucks a pound fiji i mostly covered it up.
the yellow arrow pointing at what i think you are referring to, is actually a 2 year study of mine on wether or not a chunk or rock i got out of a remote cove in cabo san lucas would effect my water parameters. it is commonly referred to as holey rock, and is composed of a high percentage of silicates. 2 years later, i have never been able to see any silicate changes in my water parameters... and as things arent dying, i am assuming no other toxic metals that test kits are not available for are leeching out either.
i just brought it with me because it almost took off half of my hand when a big swell smashed me into it, as i needed to show the sea that i was boss, i stole the offending protrusion immediately and inserted it into my own realm of control. now the rock is at my mercy!
it is green from green coraline algae, and the pink and purple are the more common varieties of coraline starting to outgrow the green.