What check would my PC roll to escape if he were grappled by a mimic?

Question

To escape a basic grapple, you can contest the grappler's Athletics check with an Athletics or Acrobatics ability check (PHB, pg. 195):

Escaping a Grapple. A grappled creature can use its action to escape. To do so, it must succeed on a Strength (Athletics) or Dexterity (Acrobatics) check contested by [the grappler's] Strength (Athletics) check.

The mimic's Adhesive trait (MM, pg. 220) says:

Adhesive (Object Form Only). The mimic adheres to anything that touches it. A Huge or smaller creature adhered to the mimic is also grappled by it (escape DC 13). Ability checks made to escape this grapple have disadvantage.

If a player touches a transformed mimic while inspecting it or something, since his hand is already stuck on the mimic, would he be forced to roll an Athletics skill check (regardless of his preferred skill) to escape the grapple?

Answer 1

You choose: Strength (Athletics), or Dexterity (Acrobatics)

The mimic is the grappler, because it is the one automatically grappling your character. You are the one being grappled; as per the quote in your question, you can choose to try to escape the grapple with either a Strength (Athletics) check or a Dexterity (Acrobatics) check.

What may be causing your confusion is that the mimic (like some other monsters) has a static DC for the trait that allows it to automatically grapple a creature that touches it, instead of obeying the rules for grappling used by player characters. This is explained in the "Grapple Rules for Monsters" sidebar of the Monster Manual (p. 11; emphasis mine):

Many monsters have special attacks that allow them to quickly grapple prey. When a monster hits with such an attack, it doesn't need to make an additional ability check to determine whether the grapple succeeds, unless the attack says otherwise.

A creature grappled by the monster can use its action to try to escape. To do so, it must succeed on a Strength (Athletics) or Dexterity (Acrobatics) check against the escape DC in the monster's stat block. If no escape DC is given, assume the DC is 10 + the monster's Strength (Athletics) modifier.

Once the mimic automatically grapples you after you touch it, you can escape by using your action to roll either a Strength (Athletics) check or a Dexterity (Acrobatics) check - your choice. If you get a total of 13 or more on the roll, you escape the grapple; otherwise, you remain grappled.