If you truly have the black bugs, they wont be in your sand bed. They only live on acropora, usually on the plug by the base or under the encrusting part if its just a frag.
That said flatworms can be a pain in the butt too. The small tiny ones are pretty harmless unless they reach plague proportions. A wrasse can help keep them in check. The bigger euphyllia eating flatworms are bad though, tough to get them all... Look into Flatworm Exit, it works quite well, just make sure you read a few threads about how to use it properly and make sure you do two more repeat treatments after so that you kill the babies that hatch from the eggs..
Thanks for the advice.
Having a look this morning I couldn't see any more black dots running around, and can't see anything on my 2 acro's. Will continue to monitor though. Did come across a couple other things but will make a new post for ID on that.
Re: the flatworms, yeah the ones I had one the hammers were pretty decent sized, dipped once and saw them all bail out, was gross. About a week or so later the missus noticed another one, so out they came again for another dip, only one maybe 2 fell off but I spent a good 20 minutes brushing the skeletons with a toothbrush to get rid of the egg patches I saw. Nothing since, thankfully but ever watchful!