weirdy leechy things inside filter

[Crevix]

DD was just doing a water change and rinsing the filter sponge and found what looks like leeches inside the filter box.

what the chuff are they? and what do I do about them?

They are probably some kind of planaria AKA flat worms, some are harmless to fish and some will eat eggs and/or fry. If they are small enough to get eaten by your fish then they probably will be . They usually turn up if fish are overfed, try rinsing the filter sponges a bit more often (always in tank water, never tap water) and once a day feed your fish what they can eat in 3 mins, scooping out any uneaten food.