Is this ual abuse?

Yes, it was ual abuse, but it's not your "fault" for not doing anything about it. You were a child, you didn't know what "ual abuse" was, and you were concerned for your mum's feelings, which was natural and considerate. Now you're more aware of what is and isn't appropriate. It was definitely ual abuse of a child by an adult. The child is never to blame for not complaining, or telling, or trying to force the adult to stop. Children naturally accept that adults know best, and aren't in the habit of telling them to stop. I think, although it's possible he doesn't intend to do more than text you, that there is that risk. He was an adult when he abused you, so he knew that it was wrong, and is totally to blame. I think you should tell your mum. She will probably be shocked, and feel upset and guilty, or angry, but remember that none of that is your fault, it is all his fault. Then you and your mum can decide what to do. You could try phoning Childline and asking their opinion, because they are very experienced in all sorts of child abuse, including cases like yours where you didn't tell till later. Of course it is always up to you what you decide to do, but consider everything before you make a decision. And dont feel guilty, you have only ever been the victim of a crime, and have done nothing wrong.