Cool tank, love the stock.
It looks to me like it could be either of those 2 things.
You will need to cover your heaters for sure. If the marks in question are not from a heater, there is a good chance the heaters could be an issue once the Ray gets bigger.
If the pleco has a taste for your rays slime coat, you may need to get it away from your ray. Often they won’t break this habit on their own.
One last thing you may need to consider is, are those rocks sharp? If so you may need to pull them too. Rays are always moving and getting themselves into things they probably shouldn’t. Lol. So if there is something that can harm them they most likely will find it in their endless search of food.