Ceramic cup seems to be cracked internally, is it safe to use?
That is called crazing.  It is a crack or fissure in the enamel coating on the cup, not indicative of deep structural flaws.

Your cup is unlikely to fail in the sense of completely breaking due to the craze in the glaze.

On the other hand, they will stain over time, and be unsightly, and hard to wash out.  

If the piece is old enough, the glaze may contain lead, which can leach due to the crazing.  In that case, the piece should definitely be discarded.

Technically, most health codes require crazed dishware to be discarded, although you can assess your tolerance for risk.