Hard skin bottom of heel from using foot massage device

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Few months ago I bought a foot massage machine, I slip my foot in and balls massage bottom of my foot.
(For some reason I cannot upload photo here), so Google "Beetle foot massage" image.
Balls circulate on front part of foot but balls move back and forth on bottom of heel.

I use it several times a day for 15-20min each time. Now after few months I noticed that middle of bottom part of heel area is developing hard skin, and maybe forming callus. I hope not. Maybe due to back and forth movement of massage balls.
Now I don;t have any hard skin on front of foot area. Maybe cus balls circulate instead back and forth.
I can feel hardness on bottom of heel when I walk bare foot in tile, but not hurting.

Question is there any concerns? Is this normal to live with hard skin?
Since hard skin or possible callus area don;t get same blood supply as soft skin area, I don;t want my heel area to suffer.