You most likely have aphids. These bugs are the ban of hibiscus growers.
Luckily, they're not a big problem, you can spray them with a soap solution. You'll need to do it almost everyday and check the plants constantly through out the winter to keep the little buggers at bay.
If it's feasible, on warm days (if in fact you HAVE warm days up in the cold North!), you can bring the plants outside, lay them on their sides and spray the foliage down thoroughly. As you spray, rub off the bugs from the leaves (top and bottom) and the blooms.
Good luck!
Barb

I had them on all my buds on a tree hibiscus this fall. Thought I got them all hosed off and sprayed with Killing soap but they came back a week or so later. I got fed up and chucked it out. Really T'd me off.

Tom

I had those too! I thought it was wierd because they are only on the buds.

I just hose them off.

Put it in the bathtub and rinse it well. Spray it with dish soap/water solution. Then go to your local garden center and ask for a "systemic" insecticide. You just water it in and it gets into the plant's system making it undesirable to pests.