To me this looks like tyre wear. Due to friction, the tyre material is being abraded and partly it sticks to the roller surface and partly it falls on the ground in the vicinity.
This is normal and would be reduced with special tyres for indoor trainers. But if you want to move the bike in and out it might be awkward to change them. If you are going to only use the bike inside for the winter season, it may be worth a consideration.