I've just set up a 75g, now just with LR and LS. I saw a cowrie creeping up the glass -- is this cause for concern of should I just leave it?

"Cowrie" is about as descriptive as "fish" and we can't tell anyone much from that. Depending on who you ask, "Cowrie" is the common name for any snail that has a shell which is sort of folded around itself, and has a mantle which extends to cover it's shell (or part of it) when the animal is active (this would include both the Cypraeids and the Ovulids) or specifically those snails of the genus Cypraea. In either case, despite the claims of your local pet shop, most species are predatory (feed on animal prey) rather than herbivorous (feed on algae). There are literally hundreds of species of Cypraea, and their diet range is just as wide as the number of species -- ranging from general scavengers to generalized omnivores to highly specific predators. Many different species are commonly imported for the reef trade, and most look so similar that it takes an expert to be able to specifically identify them. It is common to see gastropods are misidentified in pet shops, and unfortunately for you as a consumer, it is almost impossible to tell what the diet will be from any common name (the suppliers give the animals simple common names, and unless the pet shop staff really knows what the snails look like, they have nothing else to go on). I often see several different species in a tank with a single name on them, and all of this is working against you when you're trying to figure out what you have and how to care for the animal.

Despite that problem, if you can identify the animal there are a number of generalizations that you can safely make about the different groups. The egg & spindle cowries (Ovulids) are primarily cnidarian predators, typically specializing on one or a few species of soft corals, although many species may accept other foods (even algae) if they get hungry enough in an aquarium. For example, the Flamingo Tongue (Cyphoma gibbosum) may be one of the most commonly photographed snails in the Caribbean, and preys exclusively on the tissue of gorgonians. Likewise the "true" cowries (genus Cypraea) are frequently specialists on colonial invertebrates, such as tunicates, hydroids and especially sponges – if you count the number of cowries that consume a given prey item, sponges are certainly the winner. Although I say that these animals prefer a certain prey item, they should probably be considered omnivorous, really.

For all of the cowries for which the diet range is well known, they naturally consume a variety of animal and/or plant matter. Most species will graze anything from algae to sponges and cnidarians, but the majority of them are definitely eating a large proportion of animal prey in their normal diet. I have had cowries in my reef for years without a problem, but when adding a new fungid or anemone to the tank, the cowrie makes a bee-line for the new addition and chows into it. This just happened to me last month -- I had kept 2 money cowries in my reef tank for the past couple of years without incident, but when I recently added a slipper coral (Polyphillia talpina), one of the cowries marched right over and latched onto one of the tantacles of the coral and ripped it off. I moved it away, thinking it was just a fluke thing, but the next morning there was a patch of the coral that had been cleared of tentacles, and the cowrie was still at it -- it has been reloacted to my seahorse tank. The other cowrie paid absolutely no attention to the coral, and still hasn't...

The bottom line is that cowries have highly variable diets and there are even individual variation that comes into play. You never really know until you have the animal in your tank how it will react, or if that will change when anything new is added to the tank.