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Could of been a few things but I don't think it was a slug (As there are so salt water slugs). It was probably a nudi, there feeding habits are not that well understood and some eat coral, would need a photo for a positive ID.
Levi

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If you have some time to kill you should be able to find it here: http://www.seaslugforum.net/

However the vast majority of nudibranches dont eat algae so I wouldnt even bother looking. Consider improving your water chemistry, letting your tank mature some more (if its relatively new?) or getting some snails instead.

if you were at pices then i know what you were looking at. I had one a while back and they only eat sponge. They are really neat but even wiay a big yellow sponge it was gone in a week or so.

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Politely tell your LFS that they shouldn't be importing species that have little chance of success in hobbyists tanks.
A lot of newbies would buy them simply because of their colouring.

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so nudi-branches are not reef safe? just curious

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