Well, we know toddlers often bite. My question is where is the bite mark? I can't tell from your photo what part of the body that is on - I'm guessing it is on a part of Liam's body that is normally covered by his clothing, hence your discovering it at bathtime? And that could be why your DCP didn't inform you - they didn't know it had happened. As wonderful as our 2 teachers are in DD's room, I know they have a max of 8 toddlers in their care and they can't possibly witness every single push, bite, or other transgression that may occur in tiny people who can't express themselves fully in a verbal manner quite yet.
I would see what their response is - I'd assume they would adress your concerns and tell you how they typically handle known biting incidents to ensure the safety of the kids / address it with the biter (shadowing for triggers, having the biter apologize and help get ice for the bitee, separation of the kids, for example).
I would not automatically jump to the conclusion that this is somethign to pull him over.