Mantis is highly unlikely unless there was a lot of early CUC members missing(could have been a baby and eating crabs and snails and now has grown), went too long before they started missing unless there was new rock added. Look for burrows around the rock work, they typically like to have multiple entrances and exits to their lairs.
Also do not run Phosban other than for a few weeks, its a pain in the *** to constantly change, it is great to start as it will leech the Phosphate faster than algae but the issue is it becomes expired really fast(I learned this previously the hard way). What I would reccommend, run it for 2 weeks, chaning it 3 times in that period, first time 3-4 days after adding, second in a few more then leave it in a week, after that just switch to GFO as it lasts much longer.