What are these?
 

These things have been growing on one rock on one side of the tank. They are mostly flat but some of the ones located at the back of the rock extend a tube-like. Are they of concern? If so, how should I get rid of them?

I believe they are green button polyps , my tank is full of them.

I can't really tell from the picture but they're either aptasia (bad) or they are a brown palythoa (OK). Are they a solid colour or kind of translucent?

looks like palys

+1 polups OK.

Def not aptasia or green buttons my votes for the paly's also:)

The ones in the back are extending to get better light personally I love paly's and zoa's regardless of colour:)

Thanks everyone. I'll go with the assumption they are palys as they are a solid colour. Are they detrimental to other corals nearby or next to them?

I wouldn't place any coral too close together ,as long as their not touching you should be fine. I let all my paly's and zoa's grow into each other but I keep other corals with a bit of distance. Sps would take the hardest hit IMO most likely bleach over time:)

Not really a problem at all... they're pretty benign. However, if they come into contact with SPS they can raise some hell and they can grow fast and might cover slower growing corals but really I think you'll be fine. I've never removed a paly from my tank and I'm sittin' pretty good. You never know, once you get some high light on those bad boys they might turn into something cool, that's what happened with some of my hitchhiker paly's.