Their info on H. crassicornis is incorrect then. It might be found in deeper water, but it also is found right at the surface in seagrass type areas. I've never seen one in a tidepool, but I've seen dozens at the low tide mark in mudflat type areas among the seagrass; same habitat you would find the local pipefish, which would make a great tankmate. Mudflats are, by definition, pretty low wave action areas, so current isn't all that important. Also, their diet consists mainly of hydroids, not sponges. They will eat other decaying matter (they make great octopus tankmates...clean up crew) but they do need to get some hydroids now and then. Luckily it's easy to collect some. I would only keep this one if you intend on keeping this type of biotope.
I've never heard of or seen a sea lemon...sorry!