Ok I like the concepts, but agree that the abilities could be tweaked a little.
Health seems about right. I’d be tempted to bump it up to the next tier since except that I’m going to give him some healing, so let’s leave it at 8750.
Colours are great. This is one of the few times that we might actually get abilities matching the character’s colour scheme. I still want to swap them around a bit; moving Yellow to strongest colour.

Semi-solid Mass (Active Yellow, 9 AP)
Sandman is able to absorb and incorporate sand, dirt, and even rock particles from the surrounding area into his body, rapidly reforming to recover from an attack.
Destroy up to 3 random Yellow basic tiles, gaining X health for each. Does not generate AP.
Rank 2: Can also destroy attack, protect or strike tiles.
Rank 3: For each special tile destroyed, create a Yellow protect tile of the same strength.
Rank 4: Increase health gained.
Rank 5: Destroy up to 5 tiles.
Warning! True Healing ability approaches! Don’t worry, I made cost yellow AP AND scale the strength of the heal off yellow tiles destroyed, so it evens out. Mostly.
Healing and Protection are two very “yellow” type traits, and absorbing the surrounding grains of sand (yellow tiles) to reform his body is a very Sandman type trait. I don’t know if there’s much more to explain, Sandman uses the yellow on the board to reform his body, and any special tiles are converted to enhance his mass.

Sandstorm (Active Black, 6 AP)
Sandman converts a part of his mass into a vicious sandstorm to engulf his enemies.
Deals X damage to Sandman and creates 2 Yellow attack tiles.
Rank 2: Self damage reduced.
Rank 3: Create 4 attack tiles.
Rank 4: Create 6 attack tiles.
Rank 5: Create 8 attack tiles.
Max Level: Deals 875 self-damage, creates 8 strength 109 Yellow attack tiles.
At first, I was just going to make this a Yellow version of Hailstorm, but since I want the yellow tiles to represent a part of Sandman’s mass it made sense to switch this to Black. That’s ok, because Hailstorm sets a precedent for attack tile spam. Although Sandstorm doesn’t quite fill up the board in the same way. It’s cheaper, in terms of AP cost, however it does include an additional cost to Sandman’s own health - lucky he can absorb that sand back in to heal up if he needs it. Also, this ability “creates” yellow attack tiles - it doesn’t necessarily convert existing yellow basic tiles. This means that you’ll always get 8 attack tiles added to the board; you may even get 8 yellow tiles added to the board, although if they appear next to any existing yellow tiles they may auto match. And lastly, if you haven’t already done the math, at max level his attack tiles are dealing (in total) about the same amount of damage Sandman takes in creating the Sandstorm.

Makeshift Weapons (Active Green, 10 AP)
Sandman shapes his hands into weapons to crush his enemies
Convert up to 2 friendly yellow attack tiles into strike tiles, then deal X damage.
Rank 2: Increased damage.
Rank 3: Convert up to 3 tiles.
Rank 4: Can also target friendly protect tiles.
Rank 5: Convert up to 4 tiles.
Max Level: Convert up to 4 friendly attack or protect tiles into strike tiles, then deal 2406 damage.
No need to get too fancy here. I think the pattern should be obvious by now: Sandman alters yellow tiles to fit the situation. Note that the strike tiles are created and then you hit, which is the reverse of almost every other similar ability. Obvious synergy with Sandstorm here, and I’m keeping the Strikes on yellow which means they could always end up being reabsorbed and converted into protect tiles by Semi-solid Mass.
So there you have it, Sandman puts his body on the board and shapes to fit the situation. A self-healing, tanky, brawler.