Damage to bed surface from heat or chemical or..?
The build surface on the Ender3 is a BuildTak clone. The picture is a bit unclear, but given my experience with BuildTak (clones) this certainly damage because of heat. You can, as suggested before, replace the bed surface, but I do not think it is necessary at this stage. 

Normally these surfaces do not get damaged that easily but to prolong the life try to keep the following points in mind:


Correct height between nozzle and bed.
Don't let the nozzle heat up/cool down close to the bed (for example after a failed first layer).
When using sharp tools to remove prints be careful nut to dig into the surface.
Don't use too high of a bed temperature (my BuildTak clone once had bubbles forming because the layers separated)
Clean/degrease the bed, although this is more to ensure proper bed adhesion.
I found out that if the bed stops sticking you can revive it by sanding it a bit.