Unknown Crab

I have just starting setting up my first marine tank and bought 10Kg of LR in the last few days. I have several hitchhikers. The usual bristle worms (removed a long one that was almost 2 inches - pic 364), a few worms that are only about 7mm in length, few white worms about 1cm, a tiny snail - not even 1mm. I am not too worried about these but have a few questions on the rest:

Odd type of crab, pointed head, eyes that stick out, colour blends in with the cream rock, green claws with small white end. I have spent a few hours on the internet but can not identify what type he is and how much damage he could cause if I keep him. I plan to get some fish and then eventually coral and do not want him eating the fish or coral. I have attached a picture 355 and 368, any ideas?

I also have some of the rock that it green, looks like really fine algae but the guy in the shop said its a good type?! Can this be true? Pic 367. I also have some larger type of green algae / plant, this good? Pic 366.

Also, I have a small piece of rock that looks like it contains aiptasia, based on the information I have found on the internet. Pic 363. If touched, most of them (a lot on the one small piece of rock) retreat inside. How bad are they? Will the crab eat it? Should I take it back as read its like a weed which is hard to deal with...

Sorry for all the questions and the poor picture quality, they are from my iphone!

Crab looks like a decorator crab, dont know about the algae, but some things will eat it, bristleworms are a good member of the CUC, Aiptasia are bad, you want to kill them with Aiptasia X

Thank you. Think I will keep him for a bit and see what happens when I start to introduce fish. A bit mean but he can also test the quality of the water before I introduce fish!

dixon said:

Thank you. Think I will keep him for a bit and see what happens when I start to introduce fish. A bit mean but he can also test the quality of the water before I introduce fish!

Click to expand...

A bit? :grumble:

I think I will keep him until he does any significant damage, at which point I will offer him up for free or return him to my LFS. I have been checking my water daily and all the levels seem normal so sure there is no concern to his health :)