The mutant can acquire a reference to the player at a certain range. For example, every frame (or less frequently) the range between the mutant and the player can be checked. When the mutant is within a specific range, the mutant can acquire the player as target.
How to represent that is up to you, but a short version could be:
bool isHostile = true
Entity target = player
or maybe a temporary solution:
int targetX = player.x // needs updated every frame, as player moves
int targetY = player.y
After the mutant has the player as a target, the mutant can movement can head towards the player. A piece of math called a "normalized vector" is useful for figuring out how much to move in a given direction. Without a normalized vector, you'll run into the classic problem of having a game where diagonal movement is significantly faster than left/right or up/down movement. Here is some pseudocode for determining the direction to move, and how much to move:
// xDiff and yDiff would be the length of the line between mutant and player
var xDiff = target.x - npc.x
var yDiff = target.y - npc.y
// unitVector is a line which is 1.0 units long and points from mutant to player
var unitVector = math.normalizeVector(xDiff, yDiff)
// move along the line from mutant to player
npc.x += unitVector.x * npc.speed * delta
npc.y += unitVector.y * npc.speed * delta
In the above code, target would be the player, and npc would be your mutant. You can ignore/remove the delta for now, and also type in some test numbers for speed -- but it may be something for later, a topic called "frame independent movement."
The approximate math for normalizeVector is:
length = Math.sqrt((x * x) + (y * y))
var xComponent = x / length
var yComponent = y / length
This will convert an x,y movement of [+1, +1] into something like [0.707106781186547, 0.707106781186547] which is an ugly set of numbers but carries the correct amount of movement. I'll note that this particular equation will hit an error if x and y are both 0 (it'll divide by zero). If you're using a math or game library it may already have a function like this.