Find the closest treasure

Hey, I'm trying to create a 'Pokemon Go' like app. The goal is to find the closest treasure, the app should give where to go, ex: 38m North, 8m East. On every phone, the treasure need to be at the same position, i create an algorithm that said if the coord have a treasure or not return (EndWith(X%2.2839f*(Y/Modifier * 2784.3f) % 282.234492f / 4, 3, 2)); My problem is; it's very hard for the phone to find which is the closest treasure. I've create an example just for let you know what is my goal.

    if(LastPosX != MapXPos || LastPosY != MapYPos) {
                 for(int s = 0; s < 10; s++) {
                     for(float x = MapXPos - i; x < MapXPos + i; i+=0.01f) {
                         for(float y = MapYPos - i; y < MapYPos + i; i+=0.01f) {
                             if(CoordIsDefine(x*100,y*100,1)) {
                                 closetTreasurePoint = new Vector2(x,y);
                                 IsBreak = true;
                             }
                             if(IsBreak) {
                                 break;
                             }
                         }
                         if(IsBreak) {
                             break;
                         }
                     }
                     if(IsBreak) {
                         break;
                     }
                     i++;
                 }
             }

This is very slow and it doesn't realy find the closet one...