The young player is prepared to move to London to serve as a backup for Arsenal's main centre-halves, William Saliba and Gabriel Magalhaes.
Arsenal previously made an offer of €15m, but it was rejected by Valencia.
However, the club is willing to sell Mosquera for the right price, and Arsenal is expected to return with an improved offer of €20m, which should be enough to secure the transfer.
Mosquera, a Spain U21 international, has made 104 appearances for Valencia.
