yes, mating. That is one big female.
Careful. Even similar size males can do downhill and stress with prolonged attention. This is why some tinc keepers try for 2 males and 1 female ratios.
That may be a dwarf cobalt or dis-similar morph.
I would separate them and try to get a more suitable male or two for that large female.
It doesn't take too long for dart frogs to stress and go downhill.
Time flies like an arrow. Fruit flies like a bannana