That could be an algae-eating type of nudibranch. Can't really be sure until you see what it eats.
May I ask how come you're using air stone in your tank?
Mellow, if you have Microsoft Office, then you would have Microsoft Photo Editor too as it is part of the suite. You can use it to do resizing and other basic editing stuff. Don't need to "borrow" PhotoShop. Hope this helps.
I've always wondered ... how do they keep the water conditions stable (eg. SG, pH, etc) in that outdoor pond? Wouldn't the salinity be badly affected when it rains?