Reading up on mantis shrimp they have a long and complicated lifecycle. They have a plankton form/phase, and go through a metamorphosis, actually they go through about 7 stages before reaching sexual maturity.
Have you added any corals, live rocks, or water from a foreign source in the last 3-6 months?
I would try covering your powerhead or whatever else you may suspect in a mesh bag or pantyhose. If your powerhead is the cause this should solve it.
"The important thing is not to stop questioning. Curiosity has its own reason for existing" A. Einstein
"Life is either a daring adventure or nothing."
"We can do anything we want to if we stick to it long enough." Helen Keller