no worries.. worms and crab commonly seem in the rock. me have this problem too.. the worm will die eventually, if visible to you,,,, you might want to take it out, crab.. if too big.. advise to take out too.
For my jbj 24g cube... i only left with the sponge to trap big and dirty particles.. every week, i rinse and replace with new one. i removed the carbon and ceramic rings, replace with biohome, purigen and rowa, for rowa... advise to run with FR to run more effectively..... i connect to a chiller with the stock pump, only run when chiller is on. for ciculation.. i got a new maxijet 1100 pump, very effective with a 360 degree rotation dozzle. Water change.. 10% every weekend. My tank has been running for ard 8 months, now quite stable.. just change water every week.