No silly, that's not algae, that's an emerald crab!!
Are you talking about the blueygreen tint on the rock by him? Maybe cyano. Most often comes in reddish tints but you can see blueygreens and darker greens as well. Do a search on "cyano" on here. Is it most present in low flow areas of your tank?
If your worried about it spreading and want to get rid of it, try placing a rock or something over it for a week or so to block out it's light. This should get rid of it.