That very much looks like a maggot to me. I know those annoying buggers anywhere. Also, don't have to be carrying the fly eggs to get them. Flies get into EVERYTHING. Regardless of the area. Especially if you leave things rotting like hay and rabbit droppings.
Just be careful your rabbits fur and bum are moisture and poop free, and there are no open wounds. If there is any of this, then there is a higher risk of flystrike occurring, which you want to avoid at all costs. If you feel there is any risk of this, best to check your bun over completely, several times a day to make sure no eggs have been laid on the fur, and try and find the cause and/or resolve the issue so the fur stays dry.
No, don't use a damp rag. You don't want the fur to be damp or wet. That will just attract flies to lay eggs there. You need to use a flea comb or some other item to scrap off any eggs you find stuck on the fur.
What do you mean by dirty legs? Are they dry urine stained, wet or damp urine stained, or dirty with dried poop? Does your rabbit have some sort of medical condition or other reason, that is making it this way? Normally rabbits will be very clean. They are fastidious groomers. Usually the only reason a rabbit will have dirty fur is if there is a health problem, an environment that isn't staying clean enough(pee accidents outside litter box, litter box needing more cleaning, etc), or is an unneutered male spraying territory.
My biggest phobia are those (fly babies), anywhere..I can't even.
It's going to be hard for me if the name comes up in my notifications so please everyone don't quote me or like this post thanks
Ok now onto my comment.... Baby cornstarch and a flea comb and groom bunny daily, if you find eggs or are worried about possible infestation.
May I suggest you clean litterbox out daily (choose a time that you can do everyday) to make sure your bun doesn't experience this again.
It will also help with what you said is "dirty legs" (I don't know what you mean by that as a previous comment asked relivant questions to determine your meaning and as of yet no reply to them).
I will now share photos of my buns daily litterbox change (morning is when I have chosen) from used box to new day box
I start with enough hay at one end for them to enjoy through the day and spread a thin layer over the rest to help not poke their feet & I add hay throughout the day.
(I do purchase a bale of timothy at a time from a farm feed store, for cheeper expense and it lasts my 2 buns about 3-4 months)