Is it cheating when you tell someone whom you have had a relationship with you love them when currently they have a significant other?

Is it technically cheating? No. But it is a gray area in that you're hoping to establish a relationship with this person, either through an affair or them leaving their current partner to be with you. It doesn't hurt to look at it from the viewpoint of your ex's current significant other. How would feel about an ex of your partner declaring love for them when they fully are aware that your partner is with you?

Be aware that telling them that you still love them does not mean they will leave their current partner for you. By presenting them with your feelings now, you are putting yourself (from your ex's perspective) to be available to them regardless if your ex is in a relationship or not. Is this what you want?