Woohoo! It’s good to hear that your new seahorses are eating and that Wyatt, your stallion, got the hang of your new feeding station so quickly. Seahorses are greedy eaters, and your female may well follow Wyatt’s example and begin helping herself from the feeding station too. Once she realizes that’s where the goodies are, she won’t want the male to have all of the gourmet shrimp to himself.
If not, a good way to get your seahorses up to speed on the new feeding station is to target feed them with a turkey baster, and once they are eating from the baster well, use it to lead them to the new feeding station. The old-fashioned ones with the glass barrels work best because the seahorses can see the Mysis inside the baster all the way as it moves down the barrel and out the tip. By exerting just the right amount of pressure on the bulb, great precision is possible when target feeding with a turkey baster. By squeezing and releasing the the bulb ever so slightly, a skillful target feeder can keep a piece of Mysis dancing at the very tip of the baster indefinitely, and hold the tempting morsel right in front of the seahorse’s mouth as long as necessary.
If you can do that, it is an easy matter to hold a morsel of Mysis at the end of the baster, and use this tantalizing tidbit to lure the seahorse toward the new feeding trough by holding it just out of reach and leading the hungry seahorse in the direction you want her to go before you allow her to take the bait. This may have to be done in several steps, and it may take a while for you to get the seahorses accustomed to taking food from the baster before you start making much progress, but eventually you’ll have the pupil perched close enough to the new feeder for you to drop the dangling Mysis inside the feeding station before you allow them to slurp it up. This method takes time and patience, but it allows you to make sure the seahorses are getting plenty to eat while they make the transition to the new feeding station. And it’s a gradual conditioning process that will eventually work with even the slowest learners.
Yes, you did indeed see your arrow crab (Stenorhynchus seticornis) do exactly what you thought it did, Yvonne! The "trunk" that the crab opened and closed on its underside was actually its telson or tail flap, which they ordinarily keep tucked up tightly against their carapace on the underside of their bodies (cephalothorax). In males, the telson is narrow and pointed, but in females it is broad and rounded, forming a kind of apron which protects their egg masses when they are "in berry." I suspect that your arrow crab is actually a female, and that she was cleaning up beneath this apron in order to prepare it to receive her eggs eventually.
With their armored exoskeletons and the way they move around so stiffly and rigidly on their enormously elongated, stilt-like legs, arrow crabs always seem more mechanical to me than actual living, breathing animals, and when they lower their tail flap and reach up inside with their delicate pincers and start rummaging around in the opening, they always remind me of an incredibly emaciated robot lowering its access hatch and tinkering around inside itself — tightening a bolt here, flipping a switch there, and adjusting the gears in its internal workings, as it gives itself a quick tuneup! When they’re done with their fine tuning and they close the "trunk" again, you almost expect them to be holding a screwdriver in one pincer and a crescent wrench in the other. It’s quite a sight to behold.
Keep a close eye on the arrow crab to make sure it doesn’t molest your live corals, Yvonne, since they can be tough on sessile invertebrates.
Best of luck with your new seahorses! Here’s hoping that your female soon joins her partner at the feeding station.