Hi. It will be difficult without a picture (and sometimes even with one), however, I did find a picture online for you to look at. It is listed below. It is the Acropora animal question 9/20/05.
And, although these next two links are not pictures of purple crabs, they do have red eyes. The first is a good picture of the eyes, and the second is a picture of what it looks like out of the hiding spot.
http://mangrove.nus.edu.sg/pub/seashore/text/216.htm
http://www.wildsingapore.com/riablog/photos/040718-19sen/photos/sen04071814.jpg

Hope this helps.
Crab ID

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Phishie

"An ounce of prevention is worth a pound of cure."