Yikes... that's a sea spider.... at least you captured an egg-carrying one. Is your tank LPS/SPS dominant? I've found these will eat pretty much every LPS/SPS coral, so dont assume a particular coral is safe, ive found these on every coral ranging from acanthophyllia to cyphastrea, even on hydroids. They definitely do not primairy eat acans. They're primarily nocturnal, so I recommend watching your tank ~30 min after its gone completely dark, check the tank a few times a night 1+ hours apart. Sadly they always come in groups 2+ per coral, so you probably have many more of these. Look around the edges/base of coral, where the flesh meets the skeleton/rock. A good indicator of their presence is dead skeleton on the perimeter of coral, and also mesenterial filaments in the site that's being attacked. Any mesenterial filaments that are sea spider caused will disappear in a day so if you keep seeing them than you haven't gotten all of them. They also seem to be more active when food or phyto is added to the tank so I will add coral food/phyto, wait about 30 min and then scan my tank with a flashlight. Make sure to check EVERY coral, as they will infest your tank. I've taken 10+ of these off some individual coral so don't underestimate them. I grab them with needle-tip forceps, make sure to get the body and not just the legs, as they can regenerate. If you see any recession forming at the underside/edge of the coral while the rest looks healthy, it's most likely sea spiders. I've taken 40+ of these off my reef in the last 2 weeks, approaching 50, honestly have lost count. My mistake was assuming I got all of them 1 month back, they're active most on dark nights so if you have the lunar cycle feature on your lights, you won't see many during the full moon, though you can turn off the light for one night and they will start to be active. Good luck my friend, pray for the best, expect the worst.
