The 19-year-old fired into the top corner from long distance to level the score seconds after coming on for Zaine Francis-Angol on the left wing midway through the second half.
Moore, who was on the books of Hearts for six years until being released following under-15 level, struck seven goals in 12 games on loan at First Division Cowdenbeath last season and he took his form into the top flight after previously making two substitute appearances for Motherwell in cup competitions.
McCall said: "Bob McHugh has done really well. He came on against Ross County and scored and scored two in the under-20s last week but Bob's not one who can play wide.
"I think it was a bit of a surprise to the bench that we threw wee Moorey on ahead of him.
"But he can cut in off the left and he has a terrific shot with his right foot.
"He deserved his chance because he went on loan last season and scored seven goals and he has done really well this season.
"He has had to bide his time because John Sutton and Henri Anier have formed a great partnership and Bob McHugh has done well when asked. Wee Moorey is knocking on the door to be involved.
"He has a fantastic appetite for the game and a great attitude, and I was so pleased for him and his family that his first touch in league football ended up in the net.
"I just hope he is fit for the Youth Cup tie up at Montrose, he has got a bit of a knock and has ice on his knee."
It was a thoroughly miserable afternoon for Hearts manager Gary Locke, who saw his side lose the lead given to them by Ryan Stevenson's breakaway goal in the 62nd minute.
Victories for Kilmarnock and St Mirren saw Hearts fall 13 points adrift at the foot of the table and they lost Callum Paterson in the 55th minute when he went over on his ankle.
Locke said: "It's a hard one to take. I seem to be saying the same things every week.
"I certainly felt we played well enough, playing against a really good side and we more than matched them for 90 minutes.
"We went a goal ahead and at that stage I thought we were playing well, passing the ball well and getting the ball forward well. We looked really dangerous on the counter-attack.
"There is nothing you can do about the first goal, it's probably the best substitution Macca will make all season, the boy comes on and first kick of the ball, 30 yards, top corner.
"But I'm disappointed with the second one, we have got to defend corners better than that."
Goalkeeper Gunnar Nielsen also made his league debut for Motherwell after Lee Hollis pulled out through injury, and pulled off an impressive double save from Jamie Walker and David Smith just before the break.
"It's a little ankle ligament injury," McCall said. "He might be okay for next week but it will be touch and go."