i had a crab show up in mine i took it out and moved it to my refugium. the inlet tube for my HOB
fuge broke and i left it off for soemthing liek 6 months. when it broke i put all my chaeto in a 5 gallon bucket with air stone until i set it back up again, and found that the crab had wondered into the bubble chamber section after the skimmer and had no way of getting back and it died there.
now i set the fuge back up last week and everything with just chaeto, but to my surprise i found another of those crabs hidden in the shadows. my tank has been running for 2.5 years.
critters can grow and form in hidden caves in rocks and stay their untill they get bigger, i noticed some peanut worms in my rock a few months after my tank was setup and thats when i also notices the first of this type of crab, so my suggestion is if you have a refugium any critters like crabs or snails that you find that you are un aware of how they will be in youre display put them in your fuge they will also help keep the macroalgae trimmed and controlled