They look more clear in macro than they are to the naked eye. They're small enough that what colour they appear to be can be dependent on what's in their stomach at that particular moment.
In any case, none of the scary things that you need to worry about look or behave the way you described as far as I know. It's not a flat worm of any kind, it's not a nudibranch (coral eating or otherwise), it's not an acro eating 'red bug' as a) it wouldn't be on the plug and b) they're way smaller and shaped different, and it's not any of the other nasty snails or, worms, or starfishes that are known to cause problems. Even if it's not the exact type of pod from the picture, it's more than likely just one of the many beneficial/neutral microfauna that you couldn't keep out of a reef tank if you tried. I can count 5 different species of tiny crustaceany pod thingy hiding between the glass and my frag rack right now.
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