Is this Plankton, i am new to all of this
 

Any idea what this is? It looks like a spider, there are a few of them stuck to the walls of my coral reef tank? Could they have been inhabiting the living rock? We only set it up a couple of weeks ago and are still learning rapidly!

My guess is plankton, they look like spiders at first but they seem to have loads of legs?!

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I did some research online, and your picture looks just like plankton. Wish I could tell you more, but I can say with 99% sureness that what you have is plankton.

I cant really say more than that, as I deal in only freshwater. But I would assume plankton might not be that horrible in your tank if you have fish that like eating it??

Oh, and from what I think I understood from an article I read, if I remember correctly, those "legs" might be eggs??? Not sure, but I read something along the lines about a lot of eggs.

I would guess hydroid. nothing to worry about