ya i had this same problem in my 125! i got this coral and didnt dip it in anything (a green star polyp with TONS of all sorts of critters in it) anyways the 3 frags i got from that store in that tank all had some flat worms that i saw the next couple days. i didnt think they were living until i saw a snail that was on the rock almost touch one and then it scooted a little bit down farther. Thats when i got on cuz i wanted to know how fast they spread.
Well anyways i think i have only like 5 or so left in there that i still need to take out, but they are super easy to take out. All you need is a syringe or something like that and you can suck them right on up.
The most were on my green star polyps (the biggest rock) which i ended up finding a crab in so i took the whole rock out and fragged most of the green star polyps onto a different rock and then smashed the other cuz the crab wouldnt come out. (now i cant find the crab in my fuge..) But after doing that i havnt seen any on the green star polyps and ive seen 2 that i know of still on my red mushroom rock so i will take that out today and shake it in some water and then put it back in.
I suggest that you get an airline tubing since u said you have a lot and suck on the other end till water is coming out and put that end in a bucket or something and look around and suck up as many as you see and do this everyday or whenever you have the chance till you dont see anymore. They really arnt that hard to get out, unless they are going all in your rock work and you have a bunch, but this way you can get a bunch out all the time depleting their population.
Anyways good luck!
Btw if your planning on adding that sixline without quarantining good luck i hope it doesnt have ich!
Ive added 5 blue green chromis, a jawfish, and a lawnmower blenny without any sign of ich so far. But i still plan on settin up a hospital tank when ever i find a good deal on them.
But think of it if you had a sixline you would actually have a fish! and then if it ended up getting ich your quarantine would be about cycled and you could just put him in there for awhile and not have to worry about your tank cuz theres no other fish in it!