Looks like rubber deposits from the belt. Is this normal or is it my enormous power?
thanks
You seeing chunks of rubber or dust?
old dirt/lube/oil from the chain would be my guess.
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quite likely - it is my gravel bike!
clean the floor or the chain your choice 
Cleaning the chain might get you a few extra watts though
For what itâs worth, I think you should always put the trainer on a mat so that this sort of crud doesnât get all over your floors. You can try a towel, if you want, but youâll likely find that it just slides on the floor, so something âstickyâ is better.
Those are bits of weakness that have left your body.
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This. That chain could use a good cleaning 
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My advice for indoor cycling drivetrain maintenance follows.
Hot dip wax is ideal but itâs a bridge too far for most. You can 90% of the way there with a drip wax. (I use Squirt, Smoov is good too)
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Drop the applicator bottle into a bowl of piping hot (not boiling) water. Let it sit while you do the next part.
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Clean your chain thoroughly with something like the Park CM-5.3 Cyclone. Wipe dry with a microfiber.
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IMPORTANT: Use a hair dryer to warm the chain up (do not use a heat gun or flame). This also dries out any left over cleaning fluid.
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Take drip wax out of the hot water bath and apply a drop per link while slowing turning the cranks. Then, rotate in the opposite direction and apply to the underside of the links.
If you warmed the chain and drip wax enough, itâll creep into all the parts of the chain you want it. Allow that to dry before riding.
I do the above about every 300km or if the chain starts squeaking loudly. Touch wood, 6000+km later, my chain still has 100% life left.
FWIW, I get the OCCASIONAL fleck of blackened wax on my floor if I donât remove excess from the chain. Iâve never had any mess resembling the photo posted above. If you really want to go down the rabbit hole: How to wax a chain: an endless FAQ - CyclingTips
As youâve started with an oiled/greased chain, you should strip it thoroughly using a degreaser or mineral spirits before converting to wax.
Simple Green âAircraft and Precision Cleanerâ (not regular Simple Green) is safe for bike frames and components and way cheaper than Park Tool chain cleaner.
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